Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the add-ons is causing the 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

If it does work in Safe-mode then disable all 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")

To cor-el: Thank you very much. It was an extension, which I have disabled. I would however like to uninstall that extension - it applies to an application I have no further use for - but the uninstall button is greyed out and the 'troubleshooting extensions and themes' article seems to be no help. Or am I missing something?

To cor-el: I've uninstalled that surplus application and the FF extension has gone too. So all seems now to be well. Thank you again for your help.

What is the name of the program or extension that was causing your problem?

That name might be helpful for another user who has that problem.

It was an AVG 2011 (the surplus app I referred to) extension. Unfortunately my brain registered which extension it was in the list, without registering its precise name. If I remember correctly, I think the word 'search' was in the name. I'm sorry I can't be more detailed. I do hope this might help the other user!

That is probably the AVG Safe Search Toolbar extension. That extension is known to cause issues with the location bar and the Enter key not working.

To cor-el and the-edmeis...

I'm practically certain you're right. Let's hope this helps the other sufferer!