Reboot the PC into safe mode w/ networking (Shut off PC and then turn back on tapping the f8 key until boot options appear) and run a quick scan using malwarebytes or superantispyware (you'll still have internet access available to download either one). If that doesn't find anything reboot the PC normally and clear all cookies then go into programs>Mozilla Firefox>Firefox in safe mode. Check the box marked as shown in the pic:
If after that the issue persist, uninstall Firefox and reinstall. That should fix the issue.
As a tech I'm always leery of viruses hence the virus scan in safe mode first...ask if you have any further questions...
