Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).

• Don't make any changes on the Safe mode start window.

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

• Use "Disable all add-ons" on the Safe mode start window to disable all extensions.
• Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

>In Firefox 4 Safe mode also disables plugins (Tools > Add-ons > Plugins) and hardware acceleration (Tools > Options > Advanced > General)

Thank you very much for your prompt reply cor-el. Followed your instructions and found a dodgy extension. (Freeradio 0.1) It enabled me to listen to Country music ( not much here in the UK) I downloaded this when I did the upgrade. Lesson learned - do one thing at a time!!

I followed the instructions to disable all my "Add-ons". I then found that when I enabled McAfee SiteAdvisor 3.3 the problem reappeared.

The iComment toolbar was causing the problem for me.