I haven't experienced this bug. Clearing the browser cache or resetting the browser to factory defaults might help... I use the portable edition of FireFox for this exact reason: being able to save multiple copies of various different browser settings and addon configurations, and, when combined with multiple sandboxes in Sanboxie, I can run them all alongside each other, and can do so without having to worry so much about potential browser security loopholes and the related internet security software packages' annoyances like overzealous warnings, false positives, bugs, etc.. I know FireFox had a lot of flakiness going on up until not too long ago, but IME this situation appears to have been remedied to satisfactory levels. I use multiple browsers, mostly in order to try to pinpoint and avoid website related troubles like browser incompatibilities/disabilities.