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

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

Thank you - that was very helpful and solved my problem.

You're welcome

Can you tell us which Add-on caused the problem or how you otherwise solved it?
It helps other who read this thread.

Hello. Yes: I went into Safe mode and found that F11 worked, then I remembered that the problem with it not working had been when I was on a specific website, megavideo.com. So I went to this site again (whilst still in Safe mode) and although F11 works on the first page of the site, once I started watching a clip, it didn't (though it worked via the View Menu). So by taking your tip to start in Safe mode, I was able to pinpoint the problem. It wasn't the add-ons that were causing the problem, just one site, which I can live with. Thank you for your help.

Keyboard shortcuts do not work if a plugin has focus and that is what most likely happened in your case. You need to click outside the player on the page or a toolbar to set the focus elsewhere and then the F11 key and other shortcut keys should work.

Thank you. Have just tried that, but no, once the megavideo.com player is playing, even if I click my cursor elsewhere, the F11 button doesn't work and I have to access Fullscreen by going to the View Menu. Not to worry though - it's only one website. Thanks again for your help.