Clear the cache and the cookies from sites that cause problems.

"Clear the Cache":

• Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"

"Remove Cookies" from sites causing problems:

• Tools > Options > Privacy > Cookies: "Show Cookies"

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

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

didn't solve my problem except when i'm in a safe mode.

that means do I have to go on a safe mode everytime I use my facebook, yahoomail, youtube, & tweeter?

Firefox Safe mode is a diagnostic mode and not meant for permanent use.

If it does work in Safe-mode then disable all extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

• Use "Disable all add-ons" on the Safe mode start window to disable all extensions.
• Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

See also:

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

I did disable all add-on on safemode, now I can use my fb, but cant watch youtube because I need to install again the Adobe Flash Player...

what if I will uninstall the firefox mozilla then later on I will install it again, what do you think sir?? thank you.

Did you only disable the extensions (Tools > Add-ons > Extensions) or the plugins (Tools > Add-ons > Plugins) as well?

Do you still see the Flash plugin in Tools > Add-ons > Plugins?

Did you find out which extension is causing the problem by enabling only one extension at the time since it worked in Save mode?