It might be that.
Check what add-ons/extensions are ruinning in Firefox (drop-down in upper left -> Add-Ons) and make sure there's nothing funky there. Also, try another browser and see if the same thing happens there. That will help see if it's FF specific, or system wide.
If that doesn't get you anywhere, jump back to a restore point on your system (a lot of programs automatically create one, and most times, Windows updates do, too.) Look for a date before the problem started. Your files will remain intact, but you'll have to re-install any software you installed during that time.
Finally, if none of that works, download Malwarebytes, AdAware and SpyBot. After installation, reboot into safe mode and run them all (sequentially, not all at once.) Trying a few virus scan softwares makes sure you plug the gaps - no piece of software catches everything.