I'm on Ubuntu, using a copy of Firefox downloaded from Mozilla, not through a package manager.
When I restarted Firefox today I got a page informing about some new features in 69.0. It seemed to me updates downloaded some time ago but Firefox didn't prompt for restart so that they could be installed, so I tested it by downloading 68.02 (the previous version), creating a new profile, leaving it to run for some time and then looking in the About dialog and there was a button saying "Restart to update" (or something to that effect). It seems again updates downloaded but Firefox didn't prompt for restart.
I think this isn't expected behavior. Does anyone know why it happens?