Hello z_smith, Thanks for contacting Mozilla Support. '''Try Firefox Safe Mode''' to see if the problem goes away. [[Troubleshoot Firefox issues using Safe Mode|Firefox Safe Mode]] is a troubleshooting mode that turns off some settings and disables most add-ons (extensions and themes). ''(If you're using an added theme, switch to the Default theme.)'' If Firefox is open, you can restart in Firefox Safe Mode from the Help menu by clicking on the '''Restart with Add-ons Disabled...''' menu item:<br> [[Image:FirefoxSafeMode|width=520]]<br><br> If Firefox is not running, you can start Firefox in Safe Mode as follows: * On Windows: Hold the '''Shift''' key when you open the Firefox desktop or Start menu shortcut. * On Mac: Hold the '''option''' key while starting Firefox. * On Linux: Quit Firefox, go to your Terminal and run ''firefox -safe-mode'' <br>(you may need to specify the Firefox installation path e.g. /usr/lib/firefox) ''Once you get the pop-up, just select "'Start in Safe Mode"'' [[Image:Safe Mode Fx 15 - Win]] '''''If the issue is not present in Firefox Safe Mode''''', your problem is probably caused by an extension, and you need to figure out which one. Please follow the [[Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems]] article to find the cause. ''To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.'' When you figure out what's causing your issues, please let us know. It might help others with the same problem. Cheers, Patrick

Thanks Patrick, I'll let you know how it goes.

I don't know why but I can't get the problem to repeat itself now - which I guess is a good thing but not very helpful! I didn't change any plugins, but it seems to now be working fine...