Ok, maybe it should work this way, but I thought otherwise, especially since last release with improvements in stability.
I'm running Portable Firefox from USB-drive and it works alright. At the end of the day I would unplug USB-drive (app is still running) and go home. Next day, the app is still there, I plug drive back in and continue working.
Some time during that (next) day Firefox app crashes, even though drive is plugged in and everything is working fine. This happens pretty much every day and I'm not sure whether it's a problem or feature by design.