--- a/modules/libpref/Preferences.cpp
+++ b/modules/libpref/Preferences.cpp
@@ -2890,16 +2890,18 @@ nsPrefBranch::DeleteBranch(const char* a
 
     // The first disjunct matches branches: e.g. a branch name "foo.bar."
     // matches a name "foo.bar.baz" (but it won't match "foo.barrel.baz").
     // The second disjunct matches leaf nodes: e.g. a branch name "foo.bar."
     // matches a name "foo.bar" (by ignoring the trailing '.').
     nsDependentCString name(pref->Name());
     if (StringBeginsWith(name, branchName) || name.Equals(branchNameNoDot)) {
       iter.Remove();
+      // The saved callback pref may be invalid now.
+      gCallbackPref = nullptr;
     }
   }
 
   Preferences::HandleDirty();
   return NS_OK;
 }
 
 NS_IMETHODIMP