diff --git a/db/sqlite3/src/Makefile.in b/db/sqlite3/src/Makefile.in
index c745170bfb68..4c713f41c2e5 100644
--- a/db/sqlite3/src/Makefile.in
+++ b/db/sqlite3/src/Makefile.in
@@ -98,7 +98,7 @@ CSRCS = \
 # REFEF_IO allows us to override IO functions, which is used in the AsyncIO
 # -DSQLITE_SECURE_DELETE=1 will cause SQLITE to 0-fill delete data so we
 # don't have to vacuum to make sure the data is not visible in the file.
-DEFINES = -DSQLITE_ENABLE_REDEF_IO -DSQLITE_SECURE_DELETE=1
+DEFINES = -DSQLITE_ENABLE_REDEF_IO -DSQLITE_SECURE_DELETE=1 -DTHREADSAFE=1
 
 ifdef ENABLE_TESTS
 ##LIBS = $(DIST)/lib/$(LIB_PREFIX)sqlite3_s.$(LIB_SUFFIX)
diff --git a/storage/public/mozIStorageService.idl b/storage/public/mozIStorageService.idl
index 01cfcfa7ace2..f220fbbe33b8 100644
--- a/storage/public/mozIStorageService.idl
+++ b/storage/public/mozIStorageService.idl
@@ -46,6 +46,11 @@ interface nsIFile;
  * a service that can create storage connections (mozIStorageConnection)
  * to either a well-known profile database or to a specific database file.
  *
+ * The implementation of this service using sqlite with the async IO module
+ * must be created the first time on the main thread. If you want to use it
+ * from another thread, you should be sure you call getService from the main
+ * thread before accessing it from another thread.
+ *
  * This is the only way to open a database connection.
  */
 [scriptable, uuid(a4a0cad9-e0da-4379-bee4-2feef3dddc7e)]
@@ -54,7 +59,11 @@ interface mozIStorageService : nsISupports {
    * Get a connection to a named special database storage.
    *
    * @param aStorageKey a string key identifying the type of storage
-   * requested.  Valid values include: "profile", "memory"
+   * requested.  Valid values include: "profile", "memory".
+   *
+   * @see openDatabase for restrictions on how database connections may be
+   * used. For the profile database, you should only access it from the main
+   * thread since other callers may also have connections.
    *
    * @returns a new mozIStorageConnection for the requested
    * storage database.
@@ -76,14 +85,11 @@ interface mozIStorageService : nsISupports {
    * a connection to "Foo.sqlite" and "foo.sqlite" will CORRUPT YOUR DATABASE.
    *
    * Opening connections from more than one thread to the same database will
-   * also CORRUPT YOUR DATABASE. Accessing a connection object from any thread
-   * other than the thread that created it is not supported by sqlite.  Some
-   * old Linux systems might corrupt the database due to locking issues in this
-   * case.
+   * also CORRUPT YOUR DATABASE. All connections share the same sqlite cache,
+   * and the cache is not threadsafe.
    *
-   * Opening a connection from any thread but the main thread has not been
-   * tested and should be assumed to not work. Even if you find that it works,
-   * there may be subtle locking problems, so don't do it.
+   * The connection object returned by this function is not threadsafe. You must
+   * use it only from the thread you created it from.
    *
    * @param aDatabaseFile a nsIFile of the database to open.
    *
diff --git a/storage/src/mozStorageService.cpp b/storage/src/mozStorageService.cpp
index add5a3d9fde8..408bc47bd3e8 100644
--- a/storage/src/mozStorageService.cpp
+++ b/storage/src/mozStorageService.cpp
@@ -41,6 +41,7 @@
 #include "mozStorageService.h"
 #include "mozStorageConnection.h"
 #include "nsCRT.h"
+#include "nsIThread.h"
 #include "plstr.h"
 
 #include "sqlite3.h"
@@ -66,6 +67,14 @@ mozStorageService::~mozStorageService()
 nsresult
 mozStorageService::Init()
 {
+    // The service must be initialized on the main thread. The
+    // InitStorageAsyncIO function creates a thread which is joined with the
+    // main thread during shutdown. If the thread is created from a random
+    // thread, we'll join to the wrong parent.
+    NS_ENSURE_STATE(nsIThread::IsMainThread());
+
+    // this makes multiple connections to the same database share the same pager
+    // cache.
     sqlite3_enable_shared_cache(1);
 
     nsresult rv;