There seem to have been loads of these recently – it happened to me yesterday. To remedy, I summoned Task Manager with CNTL-ALT-DELETE. If I restarted Firefox normally, the red screen came back. However, the easiest way to get rid of it was to hold the SHIFT key while starting Firefox. This refreshes it, but also loses browsing history, saved passwords (together with sites where you have previously elected not to save the password) and autofill info. Somewhat irritating, but better be safe… It didn’t seem to affect my PC otherwise – a virus scan revealed nothing and it has behaved normally since.
Many people will not know how to deal with this problem, so would normally look to Google for answers. But if Firefox is your only browser, you have problems! Just as well I also have Chrome… Interestingly, I had only recently updated Firefox, so am wondering if the update was dodgy – obviously I only updated from Mozilla….
A bit more worrying is what if it happened to my Android tablet. No CNTL-ALT-DELETE option here!