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).

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.

You can use "Disable all add-ons" on the Safe mode start window to disable all extensions.

You have to close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

You may also need to delete extensions.sqlite in the Firefox Profile Folder

I noticed that Foxit reader installed the ASK tool bar (even though I chose not to have it installed) after uninstalling it from the Addons area the error stopped.

It was Ask tool bar for me too

Same for me--uninstalling it made the problem go away.

Many thanks to one and all it was the Ask and Yahoo toolbars that seem to have caused it, even though, like many others I had said I don't want it, it was installed. So thanks all problem now solved. Keith