hello qwermans, this is usually caused by outdated addons, firebug would be one of them - in case you have it installed please update it to the latest version.

Start Firefox in Safe Mode 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).

• Do NOT click the Reset button on the Safe Mode start window or otherwise make changes.

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

i tried to deactivate all addons, then i activated an addon, and +1 addon and +1 addon... finally i found the problem. it is by firebug addon. Thanks