hello, can you try to replicate this behaviour when you launch firefox in safe mode once? if not, maybe an addon is interfering here...

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Try to close all open windows and open a new window for the tabs.
If that doesn't help then quit Firefox after closing all windows and restart the Firefox application.

I had this problem too, when I disabled the TextArea Cache add-on, the problem disappeared. I'm running firefox 20,0

In my case I had to disable toolbar MozBar 2.3 (Firefox 20.0.1, WindowsXP).

I'm having this problem, but I don't have the textarea cache add-on, nore the MozBar 2.3. Now that I think about, I did just update an add-on (imtranslator), maybe that's what causing this to happen for me..

edit: yep, disabling it fixed the problem for me.