I was messing around with tab settings in Firefox about 3 years ago and screwed up my tab bar. I now have a useless blank tab bar with some half-hidden icons. I've tried to fix it several times.
The problem stays through upgrades and even a reinstall (obviously I didn't uninstall it fully). This is on a Mac, but I also did the same thing to a PC I no longer have. I think I was trying to set the tabs to get narrower instead of having to scroll to find all the tabs. I've searched through the
about:config screen for values that were "user set" but I can't find anything that affects the tab bar.
Check out the screen shot and notice the tab bar: