This issue can be caused by the McAfee Site Advisor extension

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

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

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

I have just disabled siteadvisor from my extensions in add-ons and now it seems to be O.K. This problem started after switching to FF8. When I went to get SiteAdvisor running I found out that Muck Afee is now charging for it, even though I have McAf from my ISP, as part of my subscription. Now am looking round for a good free AV as McAfee is rubbish anyway. Have had to do a complete fresh install of my XP every 4 - 6 months. I think I was lulled into a false sense of security with the SiteAdvisor from FucAmee.

Yes, disabling McAfee Site Advisor worked for me - thanks! Re-start Firefox after disabling. I re-enabled SA and restarted FF and both are working fine now.