diff --git a/extensions/help/resources/locale/en-US/cs_nav_prefs_advanced.xhtml b/extensions/help/resources/locale/en-US/cs_nav_prefs_advanced.xhtml
index 5ef81e0469d9..9ae3ffaf3362 100644
--- a/extensions/help/resources/locale/en-US/cs_nav_prefs_advanced.xhtml
+++ b/extensions/help/resources/locale/en-US/cs_nav_prefs_advanced.xhtml
@@ -375,8 +375,12 @@ you&apos;re not already viewing it, follow these steps:</p>
 </ol>
 
 <p>The Mouse Wheel preferences panel allows you to control how the mouse wheel
-  on your mouse (in between your mouse buttons) is used in
-  &brandShortName;:</p>
+  on your mouse (in between your mouse buttons) is used in &brandShortName;.
+  Modern mice may have two wheels or a button that can be used to switch the
+  scroll direction of the wheel. The behaviour for the vertical wheel function
+  is set in the upper panel <strong>Vertical scrolling</strong> while the 
+  horizontal mode is controlled by the lower panel <strong>Horizontal
+  scrolling</strong>.</p>
 
 <ul>
   <li>Select <q>No modifier key</q> or select a key that you want to use
@@ -385,16 +389,18 @@ you&apos;re not already viewing it, follow these steps:</p>
     
     <ul>
       <li><strong>Scroll the document by</strong>: Choose this to scroll the
-        document by the number of lines typed in the field.
+        document by the number of lines (or characters for horizontal movement)
+        typed in the field.
         <ul>
           <li><strong>Use system default</strong>: Choose this to use your
-	    system&apos;s default setting&mdash;the number of lines you may
-	    have previously entered will be overridden.</li>
+            system&apos;s default setting&mdash;the number of lines (characters)
+            you may have previously entered will be overridden.</li>
         </ul>
       </li>
-      <li><strong>Scroll a page up or a page down</strong>: Choose this to
-        scroll up or down one page at a time. This setting allows faster, but
-	less accurate scrolling through a page with your mouse wheel.</li>
+      <li><strong>Scroll a page up (left) or a page down (right)</strong>:
+        Choose this to scroll up or down one page at a time. This setting allows
+        faster, but less accurate scrolling through a page with your mouse
+        wheel.</li>
       <li><strong>Move back and forward in the browsing history</strong>:
         Choose this to use the mouse wheel to navigate back or forward to
 	previous pages you&apos;ve visited.</li>
@@ -407,6 +413,9 @@ you&apos;re not already viewing it, follow these steps:</p>
     <p><strong>Note</strong>: Each modifier key can be assigned to a different
       function.</p>
   </li>
+  <li>If your mouse does not have a mode for horizontal scrolling, any setting
+    in the lower panel <strong>Horizontal scrolling</strong> will be ignored.
+  </li>
 </ul>
 
 <p>[<a href="#advanced_preferences">Return to beginning of section</a>]</p>
diff --git a/xpfe/components/prefwindow/resources/content/pref-mousewheel.xul b/xpfe/components/prefwindow/resources/content/pref-mousewheel.xul
index 0841e98d9f69..be5812a9cdcc 100644
--- a/xpfe/components/prefwindow/resources/content/pref-mousewheel.xul
+++ b/xpfe/components/prefwindow/resources/content/pref-mousewheel.xul
@@ -23,6 +23,7 @@
  Contributor(s):
    bryner@brianryner.com
    sspitzer@netscape.com
+   Peter Weilbacher <mozilla@weilbacher.org>
 
  Alternatively, the contents of this file may be used under the terms of
  either of the GNU General Public License Version 2 or later (the "GPL"),
@@ -56,7 +57,11 @@
   var _elementIDs = ["mousewheelWithNoKeyAction", "mousewheelWithNoKeyNumlines", "mousewheelWithNoKeySysNumlines", 
                      "mousewheelWithAltKeyAction", "mousewheelWithAltKeyNumlines", "mousewheelWithAltKeySysNumlines", 
                      "mousewheelWithCtrlKeyAction", "mousewheelWithCtrlKeyNumlines", "mousewheelWithCtrlKeySysNumlines", 
-                     "mousewheelWithShiftKeyAction", "mousewheelWithShiftKeyNumlines", "mousewheelWithShiftKeySysNumlines"]; 
+                     "mousewheelWithShiftKeyAction", "mousewheelWithShiftKeyNumlines", "mousewheelWithShiftKeySysNumlines",
+                     "mousewheelHorizWithNoKeyAction", "mousewheelWithNoKeyNumchars", "mousewheelWithNoKeySysNumchars", 
+                     "mousewheelHorizWithAltKeyAction", "mousewheelWithAltKeyNumchars", "mousewheelWithAltKeySysNumchars", 
+                     "mousewheelHorizWithCtrlKeyAction", "mousewheelWithCtrlKeyNumchars", "mousewheelWithCtrlKeySysNumchars", 
+                     "mousewheelHorizWithShiftKeyAction", "mousewheelWithShiftKeyNumchars", "mousewheelWithShiftKeySysNumchars"]; 
    
   function switchPage( aElement )
   {
@@ -75,8 +80,10 @@
   
   function Startup()
   {
-    var fields = ["mousewheelWithNoKeyNumlines", "mousewheelWithAltKeyNumlines", "mousewheelWithCtrlKeyNumlines", "mousewheelWithShiftKeyNumlines"];
-    var checkboxes = ["mousewheelWithNoKeySysNumlines", "mousewheelWithAltKeySysNumlines", "mousewheelWithCtrlKeySysNumlines", "mousewheelWithShiftKeySysNumlines"];
+    var fields = ["mousewheelWithNoKeyNumlines", "mousewheelWithAltKeyNumlines", "mousewheelWithCtrlKeyNumlines", "mousewheelWithShiftKeyNumlines",
+                  "mousewheelWithNoKeyNumchars", "mousewheelWithAltKeyNumchars", "mousewheelWithCtrlKeyNumchars", "mousewheelWithShiftKeyNumchars"];
+    var checkboxes = ["mousewheelWithNoKeySysNumlines", "mousewheelWithAltKeySysNumlines", "mousewheelWithCtrlKeySysNumlines", "mousewheelWithShiftKeySysNumlines",
+                      "mousewheelWithNoKeySysNumchars", "mousewheelWithAltKeySysNumchars", "mousewheelWithCtrlKeySysNumchars", "mousewheelWithShiftKeySysNumchars"];
     for( var i = 0; i < checkboxes.length; i++ )
     {
       var currEl = document.getElementById( checkboxes[i] );
@@ -114,88 +121,206 @@
     <tabpanels>
       
       <!-- no key modifiers -->
-      <radiogroup id="mousewheelWithNoKeyAction"
-                  prefstring="mousewheel.withnokey.action">
-        <hbox align="start">
-          <radio value="0" label="&scroll.label;" accesskey="&scroll.accesskey;"/>
-          <vbox align="start">
-            <hbox align="center">
-              <textbox id="mousewheelWithNoKeyNumlines" size="3" 
-                         preftype="int" prefstring="mousewheel.withnokey.numlines"
-                         prefattribute="value"/>
-              <label value="&scrollLines.label;"/>
+      <vbox>
+        <groupbox>
+          <caption label="&mousewheelGroup.label;"/>
+          <radiogroup id="mousewheelWithNoKeyAction"
+                      prefstring="mousewheel.withnokey.action">
+            <hbox align="start">
+              <radio value="0" label="&scroll.label;" accesskey="&scroll.accesskey;"/>
+              <vbox align="start">
+                <hbox align="center">
+                  <textbox id="mousewheelWithNoKeyNumlines" size="3" 
+                           preftype="int" prefstring="mousewheel.withnokey.numlines"
+                           prefattribute="value"/>
+                  <label value="&scrollLines.label;"/>
+                </hbox>
+                <checkbox id="mousewheelWithNoKeySysNumlines" label="&useSystemDefault.label;"
+                          accesskey="&useSystemDefault.accesskey;"
+                          prefstring="mousewheel.withnokey.sysnumlines"
+                          oncommand="enableField(this, 'mousewheelWithNoKeyNumlines', true);"/>
+              </vbox>
             </hbox>
-            <checkbox id="mousewheelWithNoKeySysNumlines" label="&useSystemDefault.label;" accesskey="&useSystemDefault.accesskey;"
-                      prefstring="mousewheel.withnokey.sysnumlines"
-                      oncommand="enableField(this, 'mousewheelWithNoKeyNumlines', true);"/>
-          </vbox>
-        </hbox>
-        <radio value="1" label="&scrollPgUpPgDn.label;" accesskey="&scrollPgUpPgDn.accesskey;"/>
-        <radio value="2" label="&history.label;" accesskey="&history.accesskey;"/>
-        <radio value="3" label="&textsize.label;" accesskey="&textsize.accesskey;"/>
-      </radiogroup>
-        
+            <radio value="1" label="&scrollPgUpPgDn.label;" accesskey="&scrollPgUpPgDn.accesskey;"/>
+            <radio value="2" label="&history.label;" accesskey="&history.accesskey;"/>
+            <radio value="3" label="&textsize.label;" accesskey="&textsize.accesskey;"/>
+          </radiogroup>
+        </groupbox>
+        <groupbox>
+          <caption label="&mousewheelHorizGroup.label;"/>
+          <radiogroup id="mousewheelHorizWithNoKeyAction"
+                      prefstring="mousewheel.horizscroll.withnokey.action">
+            <hbox align="start">
+              <radio value="0" label="&scroll.label;" accesskey="&scrollHoriz.accesskey;"/>
+              <vbox align="start">
+                <hbox align="center">
+                  <textbox id="mousewheelWithNoKeyNumchars" size="3" 
+                           preftype="int" prefstring="mousewheel.horizscroll.withnokey.numlines"
+                           prefattribute="value"/>
+                  <label value="&scrollChars.label;"/>
+                </hbox>
+                <checkbox id="mousewheelWithNoKeySysNumchars" label="&useSystemDefault.label;"
+                          accesskey="&useSystemDefaultHoriz.accesskey;"
+                          prefstring="mousewheel.horizscroll.withnokey.sysnumlines"
+                          oncommand="enableField(this, 'mousewheelWithNoKeyNumchars', true);"/>
+              </vbox>
+            </hbox>
+            <radio value="1" label="&scrollPgLtPgRt.label;" accesskey="&scrollPgLtPgRt.accesskey;"/>
+            <radio value="2" label="&history.label;" accesskey="&historyHoriz.accesskey;"/>
+            <radio value="3" label="&textsize.label;" accesskey="&textsizeHoriz.accesskey;"/>
+          </radiogroup>
+        </groupbox>
+      </vbox>
+          
       <!-- alt modifiers -->
-      <radiogroup id="mousewheelWithAltKeyAction"
-                  prefstring="mousewheel.withaltkey.action">
-        <hbox align="start">
-          <radio value="0" label="&scroll.label;" accesskey="&scroll.accesskey;"/>
-          <vbox align="start">
-            <hbox align="center">
-              <textbox id="mousewheelWithAltKeyNumlines" size="3" 
-                        preftype="int" prefstring="mousewheel.withaltkey.numlines"/>
-              <label value="&scrollLines.label;"/>
+      <vbox>
+        <groupbox>
+          <caption label="&mousewheelGroup.label;"/>
+          <radiogroup id="mousewheelWithAltKeyAction"
+                      prefstring="mousewheel.withaltkey.action">
+            <hbox align="start">
+              <radio value="0" label="&scroll.label;" accesskey="&scroll.accesskey;"/>
+              <vbox align="start">
+                <hbox align="center">
+                  <textbox id="mousewheelWithAltKeyNumlines" size="3" 
+                           preftype="int" prefstring="mousewheel.withaltkey.numlines"/>
+                  <label value="&scrollLines.label;"/>
+                </hbox>
+                <checkbox id="mousewheelWithAltKeySysNumlines" label="&useSystemDefault.label;"
+                          accesskey="&useSystemDefault.accesskey;"
+                          prefstring="mousewheel.withaltkey.sysnumlines"
+                          oncommand="enableField(this, 'mousewheelWithAltKeyNumlines', true);"/>
+              </vbox>
             </hbox>
-            <checkbox id="mousewheelWithAltKeySysNumlines" label="&useSystemDefault.label;" accesskey="&useSystemDefault.accesskey;"
-                      prefstring="mousewheel.withaltkey.sysnumlines"
-                      oncommand="enableField(this, 'mousewheelWithAltKeyNumlines', true);"/>
-          </vbox>
-        </hbox>
-        <radio value="1" label="&scrollPgUpPgDn.label;" accesskey="&scrollPgUpPgDn.accesskey;"/>
-        <radio value="2" label="&history.label;" accesskey="&history.accesskey;"/>
-        <radio value="3" label="&textsize.label;" accesskey="&textsize.accesskey;"/>
-      </radiogroup>
+            <radio value="1" label="&scrollPgUpPgDn.label;" accesskey="&scrollPgUpPgDn.accesskey;"/>
+            <radio value="2" label="&history.label;" accesskey="&history.accesskey;"/>
+            <radio value="3" label="&textsize.label;" accesskey="&textsize.accesskey;"/>
+          </radiogroup>
+        </groupbox>
+        <groupbox>
+          <caption label="&mousewheelHorizGroup.label;"/>
+          <radiogroup id="mousewheelHorizWithAltKeyAction"
+                      prefstring="mousewheel.horizscroll.withaltkey.action">
+            <hbox align="start">
+              <radio value="0" label="&scroll.label;" accesskey="&scrollHoriz.accesskey;"/>
+              <vbox align="start">
+                <hbox align="center">
+                  <textbox id="mousewheelWithAltKeyNumchars" size="3" 
+                            preftype="int" prefstring="mousewheel.horizscroll.withaltkey.numlines"/>
+                  <label value="&scrollChars.label;"/>
+                </hbox>
+                <checkbox id="mousewheelWithAltKeySysNumchars" label="&useSystemDefault.label;"
+                          accesskey="&useSystemDefaultHoriz.accesskey;"
+                          prefstring="mousewheel.horizscroll.withaltkey.sysnumlines"
+                          oncommand="enableField(this, 'mousewheelWithAltKeyNumchars', true);"/>
+              </vbox>
+            </hbox>
+            <radio value="1" label="&scrollPgLtPgRt.label;" accesskey="&scrollPgLtPgRt.accesskey;"/>
+            <radio value="2" label="&history.label;" accesskey="&historyHoriz.accesskey;"/>
+            <radio value="3" label="&textsize.label;" accesskey="&textsizeHoriz.accesskey;"/>
+          </radiogroup>
+        </groupbox>
+      </vbox>
   
       <!-- ctrl modifiers -->
-      <radiogroup id="mousewheelWithCtrlKeyAction"
-                  prefstring="mousewheel.withcontrolkey.action" >
-        <hbox align="start">
-          <radio value="0" label="&scroll.label;" accesskey="&scroll.accesskey;"/>
-          <vbox align="start">
-            <hbox align="center">
-              <textbox id="mousewheelWithCtrlKeyNumlines" size="3" 
-                       preftype="int" prefstring="mousewheel.withcontrolkey.numlines"/>
-              <label value="&scrollLines.label;"/>
+      <vbox>
+        <groupbox>
+          <caption label="&mousewheelGroup.label;"/>
+          <radiogroup id="mousewheelWithCtrlKeyAction"
+                      prefstring="mousewheel.withcontrolkey.action" >
+            <hbox align="start">
+              <radio value="0" label="&scroll.label;" accesskey="&scroll.accesskey;"/>
+              <vbox align="start">
+                <hbox align="center">
+                  <textbox id="mousewheelWithCtrlKeyNumlines" size="3" 
+                           preftype="int" prefstring="mousewheel.withcontrolkey.numlines"/>
+                  <label value="&scrollLines.label;"/>
+                </hbox>
+                <checkbox id="mousewheelWithCtrlKeySysNumlines" label="&useSystemDefault.label;"
+                          accesskey="&useSystemDefault.accesskey;"
+                          prefstring="mousewheel.withcontrolkey.sysnumlines"
+                          oncommand="enableField(this, 'mousewheelWithCtrlKeyNumlines', true);"/>
+              </vbox>
             </hbox>
-            <checkbox id="mousewheelWithCtrlKeySysNumlines" label="&useSystemDefault.label;" accesskey="&useSystemDefault.accesskey;"
-                      prefstring="mousewheel.withcontrolkey.sysnumlines"
-                      oncommand="enableField(this, 'mousewheelWithCtrlKeyNumlines', true);"/>
-          </vbox>
-        </hbox>
-        <radio value="1" label="&scrollPgUpPgDn.label;" accesskey="&scrollPgUpPgDn.accesskey;"/>
-        <radio value="2" label="&history.label;" accesskey="&history.accesskey;"/>
-        <radio value="3" label="&textsize.label;" accesskey="&textsize.accesskey;"/>
-      </radiogroup>
+            <radio value="1" label="&scrollPgUpPgDn.label;" accesskey="&scrollPgUpPgDn.accesskey;"/>
+            <radio value="2" label="&history.label;" accesskey="&history.accesskey;"/>
+            <radio value="3" label="&textsize.label;" accesskey="&textsize.accesskey;"/>
+          </radiogroup>
+        </groupbox>
+        <groupbox>
+          <caption label="&mousewheelHorizGroup.label;"/>
+          <radiogroup id="mousewheelHorizWithCtrlKeyAction"
+                      prefstring="mousewheel.horizscroll.withcontrolkey.action" >
+            <hbox align="start">
+              <radio value="0" label="&scroll.label;" accesskey="&scrollHoriz.accesskey;"/>
+              <vbox align="start">
+                <hbox align="center">
+                  <textbox id="mousewheelWithCtrlKeyNumchars" size="3" 
+                           preftype="int" prefstring="mousewheel.horizscroll.withcontrolkey.numlines"/>
+                  <label value="&scrollChars.label;"/>
+                </hbox>
+                <checkbox id="mousewheelWithCtrlKeySysNumchars" label="&useSystemDefault.label;"
+                          accesskey="&useSystemDefaultHoriz.accesskey;"
+                          prefstring="mousewheel.horizscroll.withcontrolkey.sysnumlines"
+                          oncommand="enableField(this, 'mousewheelWithCtrlKeyNumchars', true);"/>
+              </vbox>
+            </hbox>
+            <radio value="1" label="&scrollPgLtPgRt.label;" accesskey="&scrollPgLtPgRt.accesskey;"/>
+            <radio value="2" label="&history.label;" accesskey="&historyHoriz.accesskey;"/>
+            <radio value="3" label="&textsize.label;" accesskey="&textsizeHoriz.accesskey;"/>
+          </radiogroup>
+        </groupbox>
+      </vbox>
   
       <!-- shift modifiers -->
-      <radiogroup id="mousewheelWithShiftKeyAction" prefstring="mousewheel.withshiftkey.action">
-        <hbox align="start">
-          <radio value="0" label="&scroll.label;" accesskey="&scroll.accesskey;"/>
-          <vbox align="start">
-            <hbox align="center">
-              <textbox id="mousewheelWithShiftKeyNumlines" size="3" 
-                         preftype="int" prefstring="mousewheel.withshiftkey.numlines"/>
-              <label value="&scrollLines.label;"/>
+      <vbox>
+        <groupbox>
+          <caption label="&mousewheelGroup.label;"/>
+          <radiogroup id="mousewheelWithShiftKeyAction"
+                      prefstring="mousewheel.withshiftkey.action">
+            <hbox align="start">
+              <radio value="0" label="&scroll.label;" accesskey="&scroll.accesskey;"/>
+              <vbox align="start">
+                <hbox align="center">
+                  <textbox id="mousewheelWithShiftKeyNumlines" size="3" 
+                             preftype="int" prefstring="mousewheel.withshiftkey.numlines"/>
+                  <label value="&scrollLines.label;"/>
+                </hbox>
+                <checkbox id="mousewheelWithShiftKeySysNumlines" label="&useSystemDefault.label;"
+                          accesskey="&useSystemDefault.accesskey;"
+                          prefstring="mousewheel.withshiftkey.sysnumlines"
+                          oncommand="enableField(this, 'mousewheelWithShiftKeyNumlines', true);"/>
+              </vbox>
             </hbox>
-            <checkbox id="mousewheelWithShiftKeySysNumlines" label="&useSystemDefault.label;" accesskey="&useSystemDefault.accesskey;"
-                      prefstring="mousewheel.withshiftkey.sysnumlines"
-                      oncommand="enableField(this, 'mousewheelWithShiftKeyNumlines', true);"/>
-          </vbox>
-        </hbox>
-        <radio value="1" label="&scrollPgUpPgDn.label;" accesskey="&scrollPgUpPgDn.accesskey;"/>
-        <radio value="2" label="&history.label;" accesskey="&history.accesskey;"/>
-        <radio value="3" label="&textsize.label;" accesskey="&textsize.accesskey;"/>
-      </radiogroup>
+            <radio value="1" label="&scrollPgUpPgDn.label;" accesskey="&scrollPgUpPgDn.accesskey;"/>
+            <radio value="2" label="&history.label;" accesskey="&history.accesskey;"/>
+            <radio value="3" label="&textsize.label;" accesskey="&textsize.accesskey;"/>
+          </radiogroup>
+        </groupbox>
+        <groupbox>
+          <caption label="&mousewheelHorizGroup.label;"/>
+          <radiogroup id="mousewheelHorizWithShiftKeyAction"
+                      prefstring="mousewheel.horizscroll.withshiftkey.action">
+            <hbox align="start">
+              <radio value="0" label="&scroll.label;" accesskey="&scrollHoriz.accesskey;"/>
+              <vbox align="start">
+                <hbox align="center">
+                  <textbox id="mousewheelWithShiftKeyNumchars" size="3" 
+                             preftype="int" prefstring="mousewheel.horizscroll.withshiftkey.numlines"/>
+                  <label value="&scrollChars.label;"/>
+                </hbox>
+                <checkbox id="mousewheelWithShiftKeySysNumchars" label="&useSystemDefault.label;"
+                          accesskey="&useSystemDefaultHoriz.accesskey;"
+                          prefstring="mousewheel.horizscroll.withshiftkey.sysnumlines"
+                          oncommand="enableField(this, 'mousewheelWithShiftKeyNumchars', true);"/>
+              </vbox>
+            </hbox>
+            <radio value="1" label="&scrollPgLtPgRt.label;" accesskey="&scrollPgLtPgRt.accesskey;"/>
+            <radio value="2" label="&history.label;" accesskey="&historyHoriz.accesskey;"/>
+            <radio value="3" label="&textsize.label;" accesskey="&textsizeHoriz.accesskey;"/>
+          </radiogroup>
+        </groupbox>
+      </vbox>
     </tabpanels>     
   </tabbox>
 </page>
diff --git a/xpfe/components/prefwindow/resources/locale/en-US/pref-mousewheel.dtd b/xpfe/components/prefwindow/resources/locale/en-US/pref-mousewheel.dtd
index cedc63bd6926..5bb165df5039 100644
--- a/xpfe/components/prefwindow/resources/locale/en-US/pref-mousewheel.dtd
+++ b/xpfe/components/prefwindow/resources/locale/en-US/pref-mousewheel.dtd
@@ -8,14 +8,23 @@
 <!ENTITY usingWheelAndAlt.label       "&altKey.label;">
 <!ENTITY usingWheelAndCtrl.label      "&ctrlKey.label;">
 <!ENTITY usingWheelAndShft.label      "&shiftKey.label;">
+<!ENTITY mousewheelGroup.label        "Vertical scrolling">
+<!ENTITY mousewheelHorizGroup.label   "Horizontal scrolling">
 <!ENTITY scroll.label                 "Scroll the document by ">
 <!ENTITY scroll.accesskey             "s">
+<!ENTITY scrollHoriz.accesskey        "c">
 <!ENTITY scrollLines.label            " lines. ">
+<!ENTITY scrollChars.label            " characters. ">
 <!ENTITY useSystemDefault.label       "Use system default">
 <!ENTITY useSystemDefault.accesskey   "y">
+<!ENTITY useSystemDefaultHoriz.accesskey "d">
 <!ENTITY scrollPgUpPgDn.label         "Scroll a page up or a page down">
 <!ENTITY scrollPgUpPgDn.accesskey     "p">
+<!ENTITY scrollPgLtPgRt.label         "Scroll a page left or a page right">
+<!ENTITY scrollPgLtPgRt.accesskey     "o">
 <!ENTITY history.label                "Move back and forward in the browsing history">
-<!ENTITY history.accesskey            "h">
+<!ENTITY history.accesskey            "m">
+<!ENTITY historyHoriz.accesskey       "b">
 <!ENTITY textsize.label               "Make the text larger or smaller">
 <!ENTITY textsize.accesskey           "t">
+<!ENTITY textsizeHoriz.accesskey      "e">