--- a/xpcom/ds/nsAtomTable.cpp
+++ b/xpcom/ds/nsAtomTable.cpp
@@ -517,17 +517,28 @@ NS_SizeOfAtomTablesIncludingThis(MallocS
 
   // The atoms in the this table are almost certainly stored in static data, so
   // we don't need to measure entries separately.
   *aStatic = gStaticAtomTable
            ? gStaticAtomTable->ShallowSizeOfIncludingThis(aMallocSizeOf)
            : 0;
 }
 
-#define ATOM_HASHTABLE_INITIAL_LENGTH  2048
+// The atom table very quickly gets 10,000+ entries in it (or even 100,000+).
+// But choosing the best initial length has some subtleties: we add ~2700
+// static atoms to the table at start-up, and then we start adding and removing
+// dynamic atoms. If we make the table too big to start with, when the first
+// dynamic atom gets removed the load factor will be < 25% and so we will
+// shrink it to 4096 entries.
+//
+// By choosing an initial length of 4096, we get an initial capacity of 8192.
+// That's the biggest initial capacity that will let us be > 25% full when the
+// first dynamic atom is removed (when the count is ~2700), thus avoiding any
+// shrinking.
+#define ATOM_HASHTABLE_INITIAL_LENGTH  4096
 
 static inline void
 EnsureTableExists()
 {
   if (!gAtomTable) {
     gAtomTable = new PLDHashTable(&AtomTableOps, sizeof(AtomTableEntry),
                                   ATOM_HASHTABLE_INITIAL_LENGTH);
   }