There are some repositories that still carry the deb version of firefox.
e.g ppa:mozillateam/ppa
or Ubuntuzilla
On my machine, the deb version starts up in 5 secs compared to 1 to 2 mins for the snap version 
(btw: the reason why you didn’t have to mount it before is because the smb:// url is probably a gio enabled gvfs type URI (pcmanfm can also use these) and they mount to /run/user/<your_uid>gvfs when they are first accessed / not that it helps any)