Try to disable hardware acceleration in Firefox.

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

• https://hacks.mozilla.org/2010/09/hardware-acceleration/

Thanks for the reply. This option is disabled already.

Try a clean reinstall and delete the Firefox program folder before reinstalling a fresh copy of Firefox.

Download a fresh Firefox copy and save the file to the desktop.

• Firefox 19.0.x: http://www.mozilla.org/en-US/firefox/all.html

Uninstall your current Firefox version, if possible, to cleanup the Windows registry and settings in security software.

• Do NOT remove personal data when you uninstall your current Firefox version, because all profile folders will be removed and you will also lose your personal data like bookmarks and passwords from profiles of other Firefox versions.

Remove the Firefox program folder before installing that newly downloaded copy of the Firefox installer.

• It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling.
• http://kb.mozillazine.org/Uninstalling_Firefox

Your bookmarks and other profile data are stored in the Firefox Profile Folder and won't be affected by an uninstall and (re)install, but make sure that "remove personal data" is NOT selected when you uninstall Firefox.

If you keep having problems then also create a new profile.

• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/Profile_backup

• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall

This was a clean installation of firefox 19 on a new installation of windows.. I will give it a try but I am not very optimistic.

Followed the steps one-by-one. No luck, same behaviour.

Try to boot the computer in Windows Safe mode with network support (press F8 on the boot screen) as a test to see if that helps.

Did you try a new profile?

Working with a new profile before importing the old one and without sync does not reproduce the weird behaviour. However to manually remake all bookmark entries and other settings seems very painful to me. Should I try importing all the settings sync can import without using mozbackup or it doesn't make any difference?

I created a new profile and setup sync. I had to reinstall all addons but it was worth it since the toolbar now behaves normally Apparently the profile I have been carrying a few years now with all previous versions of firefox carried some bugs with it. It could also have been a problem with the 64 bit version of windows I installed since all previous versions were 32 bit.

I had the same issue but it might not be caused by the same thing but for whoever finds this thread and with the same issue I thought I'd say that I fixed it by removing the addon called "Menu Editor". So if you have that addon along with this problem, try to disable it and see if it works.