Mozilla Firefox crashed according to a pop up. You need help fixing this problem. You do not have virus protection at this time and this is a concern.
You can clear your Firefox cache, cookies, and history. Depending on what version of Firefox your using, at a top of a Firefox window, on a menu bar, click on a "History" menu option, and select "Clear Recent History". Click a drop down menu next to "Time range to clear" to choose how much of your history Firefox will clear.
Select "Everything" if your fine with loosing all your cache, cookies, and history. Next, click an arrow next to "Details" to select exactly what information will get cleared. Select everything except "Site Preferences".
Finally, click a "Clear Now" button and a window will close and any items you've selected will be cleared. There might be some cached files that is causing Firefox to crash. Clearing all of these files can solve your problem.
Also, you can perform a repair install of Firefox if that option is available. Another possible fix is to remove Mozilla Firefox web browser and then install most recent version available.