Whether it happens to particular website or all???

Start Firefox in Safe Mode to Troubleshoot the issue and to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Many site issues can be caused by corrupt cookies or cache.

• Clear the Cache

Press the <Alt> or <F10> key to bring up the tool bar. Followed by;

Windows; Tools > Options Linux; Edit > Preferences Mac; application name > Preferences

Then Advanced > Network > Cached Web Content: Clear Now

and

• Remove Cookies

Press the <Alt> or <F10> key to bring up the tool bar. Followed by;

Windows; Tools > Options Linux; Edit > Preferences Mac; application name > Preferences

Then Privacy. Under History, select Firefox will Use Custom Settings. There is a button on the right side called Show Cookies.

If there is still a problem, Start Firefox in Safe Mode {web link} While you are in safe mode; Press the <Alt> or <F10> key to bring up the tool bar. Followed by;

Windows; Tools > Options Linux; Edit > Preferences Mac; application name > Preferences

Then Advanced > General. Look for and turn off Use Hardware Acceleration.

Poke around safe web sites and see if there is still a problem. Then restart.

Thank you for the support. Actually, the source of the problem came from the mouse itself. But what is ununderstable is that the problem appears only with Firefox. I changed the mouse and then everything is back to normal. Sorry for the disturbance. Best regards

Note that such issues can be caused by hardware acceleration in Firefox.