Try the Firefox SafeMode to see how it works there.
A troubleshooting mode, which disables most Add-ons.
Troubleshoot Firefox issues using Firefox SafeMode

• You can open the Firefox 15.0+ SafeMode by holding the Shft key when you use the Firefox desktop or Start menu shortcut.
• Or use the Help menu item, click on Restart with Add-ons Disabled... while Firefox is running.

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut (without the Shft key) to open it again.

If it is good in the Firefox SafeMode, your problem is probably caused by an extension, and you need to figure out which one.
http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+themes
Or it might be caused by Hardware Acceleration.

When you figure out what is causing that, please let us know. It might help other user's who have that problem.

Thank you for the reply.

It works fine with the safe mode. So it must be something to do with checkCompatibility 1.3 or Google Toolbar for FireFox 7.1.20110512W.

Both are necessary to run Google Toolbar on Firefox, and the fact is Google Toolbar is too convenient to be removed. So I have bookmarked a couple of blank pages and I right-click them to open new tabs. But it is so tiresome.

I hope this problem will be fixed in the next version. FireFox without Google Toolbar is just a useless browser.

Hello inu52, note that Google Toolbar it is not supported on Firefox 5 and above (it is Google choice ! ).

https://www.google.com/intl/en/toolbar/ff/index.html

http://googletoolbarhelp.blogspot.gr/2011/07/update-on-google-toolbar-for-firefox.html

thank you