Hi Mark, do you use Avast by any chance? Several Avast users are reporting similar problems today. One possible workaround is to move Firefox to a different folder. This may help, but unfortunately I cannot tell you what is going on on your system. '''''Earlier Threads''''' * https://support.mozilla.org/questions/1290476 * https://support.mozilla.org/questions/1290490 * https://support.mozilla.org/questions/1290528 * * https://support.mozilla.org/questions/1290513 * https://support.mozilla.org/questions/1290541 '''''Program Move Workaround''''' The user in that third thread reported that moving the contents of: C:\Program Files\Mozilla Firefox to C:\Program Files (x86)\Mozilla Firefox allowed Firefox to run. The user in the second thread reported that worked for them. I don't know why that would make a difference.

Hi Mark, in case you use Avast, Avast has issued an update for Avast Cleanup that should fix this: https://forum.avast.com/index.php?topic=234798.0