Probably caused by an incompatible extension.

http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+themes

Thanks Edmeister for your speedy response. I guessed it might be related to AVG safe search - I notice the latest update is incompatible with this. I'll check if safe search is disabled - but if it also means I have to disable Google toolbar then I'll have to revert to an earlier version or (as a very last resort) even to IE

Disabled AVG safe search - no longer get error message. I'm pleased to say Google toolbar still works fine (I've reported this via Add-on Compatibility Manager). Thanks very much for your help