Hi. Try pressing F9 and the Windows key simultaneously one or more times OR Try pressing Windows Key and the Left Shift key simultaneously in the same manner. Hope this helped :)

Start Firefox in Safe Mode to check if one of the extensions (Firefox menu button/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox menu button/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) to see if that helps.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

thanks for that. Starting Firefox in safe mode doesn't change anything. Starting Windows in safe mode does help but I can't run it in safe mode all the time - what's the next step? The options suggested above to hit the windows key and F9 or Left shift key did not help

You may have ad / mal-ware. Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

What security software do you have?

It is possible that your security software is causing problems. Try to disable your security software temporarily to see if that makes a difference.