The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information. Note: This will cause you to lose any Extensions, Open websites, and some Preferences.

To Reset Firefox do the following:

1. Go to Firefox > Help > Troubleshooting Information.
2. Click the "Reset Firefox" button.
3. Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
4. Firefox will open with all factory defaults applied.

Further information can be found in the Refresh Firefox - reset add-ons and settings article.

Did this fix your problems? Please report back to us!

Brilliant, thank you - this has worked. I feel stupid for not doing this previously - I'm not completely computer illiterate, but I didn't realise I could reset Firefox - I was about to reinstall.

Interestingly, I looked in the file that was created for old Firefox data, and there was the offending file, in the 'search engine' folder - it wasn't there when I was looking for it when that was the 'real' Firefox data!

Anyway, thanks again - much appreciated!