Is the Firefox window maximized?

You can try to set this pref to false on the about:config page

• browser.tabs.drawInTitlebar = false

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Thank you! This solved the issue. I guess this was caused by an add-on. Where can I see which one is responsible for it? Maybe it's worth deactivating it.

If it works in Safe Mode and in normal mode with all extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) disabled then try to find which extension is causing the problem by enabling one extension at a time until the problem reappears.

Close and restart Firefox after each change via one of these:

• "3-bar" menu button -> Exit (Power button)
• Windows: File -> Exit
• Mac: Firefox -> Quit Firefox
• Linux: File -> Quit

It is the Tab Group add-on. I will report it to have it fixed. Thanks again.

You're welcome and thanks for reporting the culprit.

I have the same problem but I LIKE my Tab Groups. Has this add-on been fixed? If so how do I get the fix? Or do I just have to give up Tab Groups?

hschmaltz said

I have the same problem but I LIKE my Tab Groups. Has this add-on been fixed? If so how do I get the fix? Or do I just have to give up Tab Groups?

It's sad, but true ..... See : http://fasezero.com/lastnotice.html