Are you sure this isn't a part of the persona you're using? Try a different skin to verify.
If problem persists, start firefox in "Safe Mode" (Press "SHIFT" on keyboard and click on firefox icon. A small popup will ask you i you want to start firefox in safe mode or reset it, select the option for safe mode).
Report back your findings
For Appearance I'm using the Default 35.0 theme.
The problem is very intermittent and seems to happen more often with 2 or more tabs open.
Could it be caused by an Extension or a Plugin?
The Plugins are set to 'Ask to Activate.'
What plugins do you have? If you have conventional ones like Flash, Java, Silverlight, etc, it's very unlikely.
But Extensions can sure be the problem. Try disabling all the extensions and check for a while if it reoccurs.
If it doesn't, we've found the source. If it does, we'll need to troubleshoot more.
I was surprised by the number of Extensions and Plugins, listed below:
Plugins: (set to 'Ask to Activate')
OpenH264 Video Codec provided by Cisco Systems, Inc.
Foxit PhantomPDF Plugin for Mozilla
Intel® Identity Protection Technology
iTunes Application Detector
Microsoft Office 2010
Nero Kwik Media Helper
NVIDIA 3D VISION
Netcraft Anti-Phishing Toolbar
Privacy Badger Firefox
Wondershare Video Converter Ultimate
But like I said, very intermittent. Last time I saw the problem was on January 4th.