In PageTriage, we have a JS file that loads a separate ResourceLoader module if needed. It uses the syntax:
mw.loader.load( 'ext.pageTriage.views.toolbar' )
In some rare circumstances, it loads the ext.pageTriage.views.toolbar module twice (although the load command is only called once), causing the interface to break badly. This seems to only happen in Firefox.
I suspect this is a similar issue to Bug 34696. The steps to reproduce are a bit complicated, so I'm not going to post them unless you need them. Just let me know.