Been running firefox 26 for a little while now with no issues.
BTW - anyone know what happened to the option NOT to always show tabs, i.e. the tab doesn't show if there's only one tab open?
You might have solved this though, in the URL field of the browser:
And scroll down to browser.tabXXXXXX
From there its a question off trial and error.
Keep in mind that Firefox is quite unstable about most changes.
Weird as it sounds, one might have to reset old config settings and try the same again.
And then it works. (Is that called a bug perhaps... though this is a well known feature in the community)
Edit: If you have not yet tried it.
Full-screen-mode is really an enhancement of Firefox anf hides title-bar and tabs in a second stage.
Just a tip for its a goodie..... ;-) [F11]