diff --git a/.hgignore b/.hgignore
index 895885254eab..7aa79886a2da 100644
--- a/.hgignore
+++ b/.hgignore
@@ -43,6 +43,7 @@ _OPT\.OBJ/
 ^js/src/autom4te.cache$
 # SpiderMonkey test result logs
 ^js/src/tests/results-.*\.(html|txt)$
+^js/src/devtools/rootAnalysis/t/out
 
 # Java HTML5 parser classes
 ^parser/html/java/(html|java)parser/
diff --git a/js/src/devtools/rootAnalysis/analyze.py b/js/src/devtools/rootAnalysis/analyze.py
index 5d41828ca02c..69482dab7b71 100755
--- a/js/src/devtools/rootAnalysis/analyze.py
+++ b/js/src/devtools/rootAnalysis/analyze.py
@@ -242,7 +242,7 @@ if args.tag and not args.buildcommand:
 if args.jobs is not None:
     data['jobs'] = args.jobs
 if not data.get('jobs'):
-    data['jobs'] = subprocess.check_output(['nproc', '--ignore=1'])
+    data['jobs'] = subprocess.check_output(['nproc', '--ignore=1']).strip()
 
 if args.buildcommand:
     data['buildcommand'] = args.buildcommand
@@ -286,7 +286,7 @@ for step in steps:
         for (i, name) in out_indexes(command):
             data[name] = outfiles[outfile]
             outfile += 1
-        assert len(outfiles) == outfile, 'step \'%s\': mismatched number of output files and params' % step
+        assert len(outfiles) == outfile, 'step \'%s\': mismatched number of output files (%d) and params (%d)' % (step, outfile, len(outfiles))
 
 if args.step:
     steps = steps[steps.index(args.step):]
diff --git a/js/src/devtools/rootAnalysis/explain.py b/js/src/devtools/rootAnalysis/explain.py
index ec7ef1949b1d..dc8b76f5c687 100755
--- a/js/src/devtools/rootAnalysis/explain.py
+++ b/js/src/devtools/rootAnalysis/explain.py
@@ -3,6 +3,8 @@
 import re
 import argparse
 
+from collections import defaultdict
+
 parser = argparse.ArgumentParser(description='Process some integers.')
 parser.add_argument('rootingHazards', nargs='?', default='rootingHazards.txt')
 parser.add_argument('gcFunctions', nargs='?', default='gcFunctions.txt')
@@ -22,7 +24,7 @@ try:
 
         # Map from a GC function name to the list of hazards resulting from
         # that GC function
-        hazardousGCFunctions = {}
+        hazardousGCFunctions = defaultdict(list)
 
         # List of tuples (gcFunction, index of hazard) used to maintain the
         # ordering of the hazards
@@ -53,7 +55,7 @@ try:
                 # Function names are surrounded by single quotes. Field calls
                 # are unquoted.
                 current_gcFunction = m.group(2)
-                hazardousGCFunctions.setdefault(current_gcFunction, []).append(line)
+                hazardousGCFunctions[current_gcFunction].append(line)
                 hazardOrder.append((current_gcFunction, len(hazardousGCFunctions[current_gcFunction]) - 1))
                 num_hazards += 1
                 continue
@@ -84,12 +86,13 @@ try:
             if current_func:
                 gcExplanations[current_func] = explanation
 
-            for gcFunction, index in hazardOrder:
-                gcHazards = hazardousGCFunctions[gcFunction]
-                if gcFunction in gcExplanations:
-                    print >>hazards, (gcHazards[index] + gcExplanations[gcFunction])
-                else:
-                    print >>hazards, gcHazards[index]
+        for gcFunction, index in hazardOrder:
+            gcHazards = hazardousGCFunctions[gcFunction]
+
+            if gcFunction in gcExplanations:
+                print >>hazards, (gcHazards[index] + gcExplanations[gcFunction])
+            else:
+                print >>hazards, gcHazards[index]
 
 except IOError as e:
     print 'Failed: %s' % str(e)
diff --git a/js/src/devtools/rootAnalysis/loadCallgraph.js b/js/src/devtools/rootAnalysis/loadCallgraph.js
index a221d7143764..7661aad8e53a 100644
--- a/js/src/devtools/rootAnalysis/loadCallgraph.js
+++ b/js/src/devtools/rootAnalysis/loadCallgraph.js
@@ -53,13 +53,8 @@ function addGCFunction(caller, reason)
 
 function addCallEdge(caller, callee, suppressed)
 {
-    if (!(caller in calleeGraph))
-        calleeGraph[caller] = [];
-    calleeGraph[caller].push({callee:callee, suppressed:suppressed});
-
-    if (!(callee in callerGraph))
-        callerGraph[callee] = [];
-    callerGraph[callee].push({caller:caller, suppressed:suppressed});
+    addToKeyedList(calleeGraph, caller, {callee:callee, suppressed:suppressed});
+    addToKeyedList(callerGraph, callee, {caller:caller, suppressed:suppressed});
 }
 
 // Map from identifier to full "mangled|readable" name. Or sometimes to a