Does anything happen..any error or just nothing happen,did you change UAC settings.

no, nothing happens.. I didn't change any settings but maybe something happened during last auto update

Hi, hold down the Shift key when you start Firefox to start in Safe Mode. More - Use Troubleshoot Mode in Firefox.

If it works in Safe Mode, disable any theme or extensions you have - 3-bar menu > Add-ons > Themes/Extensions and restart Firefox; then if it's OK, re-enable them one by one until you find the culprit. You may need to restart Firefox after enabling some extensions.

If it still won't run, try a Clean Reinstall; this means you remove your Firefox program files and then reinstall Firefox. This process does not remove your Firefox profile data (such as bookmarks and passwords), since that information is stored in a different location. With Firefox closed, use another browser to download Firefox from https://www.mozilla.org/en-US/firefox/all/ and save the file to somewhere convenient on your computer.

Now delete the Firefox installation folder which should be in one of the following locations -

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

You can now double click the file you previously downloaded to install Firefox.

If your question is resolved by this or another answer, please help other users by marking the best reply as Solved. Thank you!

thanks but nothing happens when I click firefox while holding the shift key