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.org/kb/Safe+Mode

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

See also:

• http://kb.mozillazine.org/Preferences_not_saved
• https://support.mozilla.org/kb/Preferences+are+not+saved

Already try all that with any chance

Did you try to delete the prefs.js file and a possible user.js file in the Firefox Profile Folder?

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Open Containing Folder

Now i delete these two files it keep the proxy server i choose.

So what is the wrong?

And i have install the plugin greek/english dictionary but i want to completely remove the only english dictionary and kept the greek/english dictionary but i can't.

how i do that?