Hi   !
Try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).

If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

• Click the menu button , click Help and select Restart with Add-ons Disabled.

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
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Safe Mode window appears, select "Start in Safe Mode".

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in 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.

Hi Happy112.

Thank you for your reply.

I did as you wrote, but the problem is still there. It's not going to ruin anything, it's just annoying.

\Kenneth

Bummer   !
And you did try unchecking 'Use hardware acceleration when available', then closed and restarted Firefox   ?

You may think 'it's no biggie', but this should work properly. If the above didn't solve it, let's try something else. Would you take a look at the article   'Upgrade your graphics drivers to use hardware acceleration and WebGL'     http://mzl.la/1xKsEca

I did uncheck "Use hardware acceleration", and the graphic card driver is updated. No change.

I guess the problem started, when I installed the 64 bit version of Firefox. I did not uninstall til 32 bit version though. That's the only change I can think of.

It's beginning to sound a bit SciFi-ish to me ........ Let's try this:
www.wikihow.com/Clear-the-Cache-in-Firefox

No change, sorry.

There's no problem when using Chrome and Edge I might add.

I'm giving it one more shot   (well, I'm letting you do the work) :
https://blog.mozilla.org/theden/2012/06/28/remove-a-single-website-from-your-firefox-history

Clear the cache and remove the cookies from websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Clear the cache":

• Firefox/Options/Preferences -> Advanced -> Network -> Cached Web Content: "Clear Now"

"Remove the cookies" from websites that cause problems.

• Firefox/Options/Preferences -> Privacy -> "Use custom settings for history" -> Cookies: "Show Cookies"