Linux Mint Forums

It puzzles me too
Have you made any changes to some settings - not that I know what would cause this behavior in a fresh install - that is if it is not just a one time event.
Firefox has a tendency, more in Windows than in linux, to sort of "semiopen" and you have to kill the firefox process to be able to open it. Happens once in a while. Epiphany is mozilla so if FF does something stupid it might effect Epiphany too

maybe the profile is corrupt

Maybe but I did not consider that in a fresh install. Open the profile manager - don't remember if it's FF or Thunderbird where you need to use the terminal for that - and create a mew profile and remove the old