Start Firefox in Safe Mode

At the top of the Firefox window, click the Firefox button, go over to the Help menu and select Restart with Add-ons Disabled....

Firefox will start up with the Firefox Safe Mode dialog.

Note: You can also start Firefox in Safe Mode by holding down the shift key while starting Firefox.

Clicking the Start in Safe Mode button will temporarily disable your extensions and themes, turn off hardware acceleration and reset toolbar and button customizations. When you leave Safe Mode and start Firefox up normally.

For more help: Troubleshoot Firefox issues using Safe Mode

and please also run a full scan of your system with different security tools like :

http://www.malwarebytes.org/free/

http://www.bleepingcomputer.com/download/adwcleaner/

for more information see this support article:

Troubleshoot Firefox issues caused by malware

Whenever you get a message / popup that software / files need to be updated,

DO NOT USE ANY OF THE PROVIDED LINKS.

While this may be a legitimate message, it could also be Malware or a Virus. Any time you want or need to check for upgrades, go to the web site of the True Owner of the program in question. For example, to check out Firefox, go to Mozilla.org or Firefox In Any Language.