There are plenty of web threads which suggest that you're not alone but, regrettably, there seems to be no single solution that's guaranteed to work. So let's start with the basics:
Have you checked that the date is correctly set on your computer?
Are you sure that you're using the latest version of Firefox? (It should be 43.0.4). To check, go to Help > About Firefox. (If you've not got 'Help' at the top of your screen, right-click up there and put a tick next to 'Menu Bar')