Hi,
I can't reproduce the error using Firefox 43.0.4 + Firebug 2.0.13 on Windows 7, that's why I was asking for the screenshot.
The next steps I'd try to is to have a look for the error using the built-in DevTools. Maybe they provide more info about it.
Otherwise I suggest to follow the
first aid steps to check whether your issue is related to a profile setting or a conflict with another extension (like e.g. YSlow or FireStorage Plus!, as I can see in the screenshot).
Sebastian