Could you give more information?

• Create a screenshot of the problem

If there are problems with updating or with the permissions then best is to download the full version and trash the currently installed version to do a clean install of the new version.

Download a new copy of the Firefox application and save the disk image file to the desktop

• Firefox 27.0: http://www.mozilla.org/en-US/firefox/all.html

• Trash the current Firefox application (open the Applications folder in the Finder and drag the Firefox application to the Trash) to do a clean (re-)install
• Install the new version that you have downloaded
• https://support.mozilla.org/kb/Installing+Firefox+on+Mac

Your personal data is stored elsewhere in the Firefox profile folder, so you won't lose your bookmarks and other personal data when you uninstall and (re)install Firefox.

• http://kb.mozillazine.org/Profile_folder_-_Firefox

Here's the devil. It bothers me so much! It appears everywhere and covers things

Hmm,

Maybe it is a addon or an malware, try start Firefox in Safe Mode and Scan your PC:

• Firefox in safe mode
• Problems caused by malwares

Also you can try Reset Firefox, but this action will make you lose your data, i think that you should do a clean reinstall( as cor-el said) before reset Firefox.

• Reset Firefox

Hmm Victor lol I cannot scan my PC because it's a Mac and I am using MacKeeper which will know in a second if there's any malware in the system. About addon, no, it's not because I checked my addons list and it's not there. Any other ideas? Where else to look?

Try create a new profile:

• Create a new profile

Did you at least try Safe Mode?

Did you already update to the current Firefox 27.0 release?

The Firefox version that you currently run is no longer supported with security updates.

• It is important to update Firefox and add-ons to the latest version to get all security fixes.

Cor-el no, I did not update because I didn't know about the release and I always check firefox for updates and is always telling me "your firefox is up to date"! I am gonna do it right now and let's see if this will solve the problem. By the way, Thanks both of you for support! Ill be back after update. Oh and no, didn't try the safe mode, don't know how to do it on Mac

It worked! No pop up anymore after the firefox update. Thanks so so much and hopefully the devil will not return :)