Someone with the same sort of problem.
Maybe download update and run the free version of Malwarebytes to see if it finds any problems.
If you have a Google toolbar maybe remove it.
Or try command prompt and type in ipconfig /flushdns and press enter it should tell you dns cache has been flushed if so reboot and see if it makes any difference.
I removed Firefox last week because it kept freezing on the odd page and I had to shut it down and then open it again for it to work.
Maybe download Revo Uninstaller and remove Firefox Completely.Also download the McAfee removal tool and remove that as well then reinstall it to see if it makes any difference.