duplicate of [/questions/1190156] thread. I moved over to Edge for a bit as my Firefox was running so slow. Now they have updated it, I thought I'd try it again as Edge is rubbish. I did a clean reinstall and it won't open. I have had this problem before with Chrome and I solved it by renaming the exe file and changing the shortcut. This works with firefox too, but then I can't set it as the default browser as it is no longer recognised as an app. There is obviously something, perhaps in the Windows 10 set-up, that is blocking the file firefox.exe starting, but I've not idea how to solve it. I've gone into windows defender and tried allowing it as an app, which it already was. I've tried opening it as an administrator, still nothing. I've looked through all the help threads I can find on the subject, but not got anywhere. I'm not a massive techno bod, so a simply explained answer would be best if someone can help. Thanks in advance.