Which security software (firewall, anti-virus) do you have?

If you use ZoneAlarm Extreme Security then disable Virtualization and reinstall Firefox.

Other security software may have similar features to prevent Firefox from updating files properly.

See:

• http://kb.mozillazine.org/Browser_will_not_start_up#XULRunner_error_after_an_update

Thanks for your reply But update works fine on mozilla firefox. And dont use ZoneAlarm. After cliking help, about, update, its showing up to date. eve if i uninstall and try (no deleting) same error come only when i launch it from task bar. And also when i right click from task bar, Open new tab and Enter private browsing icons are missing.

Such an error is caused by some files that weren't updated successfully, so you are left with a mixture of files from different Firefox versions.
There is usually some security software involved that protects .ini files against modification (update).

You will still have to do a clean reinstall and delete the Firefox program folder before reinstalling a new copy of Firefox.

• C:\Program Files (x86)\Mozilla Firefox\

Thank you so much bro that worked perfectly. I uninstalled both my browsers and deleted the folder too.... my water fox folder contains some exe and ini files. After the deletion i start up with a fresh installation, But some additional dialogs appeared and the app automatically removes some stuffs a completed the installation. But Open new tab, Open new window, Enter private browsing menu are missing. :). no worries coz i dont use them..

I'm not sure what you are missing.

Do you have the orange Firefox button or the menu bar?

You can check for problems with the localstore.rdf file.

• http://kb.mozillazine.org/Corrupt_localstore.rdf