hello Thomas_Hunt, can you try launching firefox in safe mode by pressing the shift key while it is starting and see if the issue is occurring there too? maybe a theme or hardware acceleration is interfering here...

if safemode is working, you can try disabling hardware acceleration in firefox > options > advanced > general and or disable addons that might cause the problem. it also would be interesting which graphics hardware (adapter description, driver date & driver version) you are using - you can access this information in firefox > help > troubleshooting information towards the bottom of that page... thank you

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

edit: from another user we found out that this might be related to malware, could you see if you have the same addons installed, remove/disable them & run a in-depth scan of your system with security software - see https://support.mozilla.org/en-US/questions/940393#answer-377118

Thanks for the help Madperson, judging from the case you linked, the common problem was the Codec 1.0. I had it as well. After removing it, Firefox was back to normal. Thanks! Thomas Hunt

thanks for reporting your findings!