Hhmm... funny this should require a plugin, it seems like a basic setting on Android phones.
Anyway: I searched the Market and there were a few paid apps for this. Then I found
Rotation Locker by madmack which is free and includes a Tasker plugin.
I was able to get what I want by setting it to landscape (checking Rotation Locker background service) when Firefox opens, then use a Tasker Exit task to set rotation to auto (and disable the service).
It feels a bit sluggish though, I don't know if that is caused by this app, Firefox, or any of the rest of the stuff running on my phone...
Thank you for your help!