The problem is that when the package manager does its updates it replaces Firefox' files while it is running, putting it into an inconsistent state, so Firefox

The problem is that when the package manager does its updates it replaces Firefox' files while it is running, putting it into an inconsistent state, so Firefox rightfully enters a defensive mode and prevents you from doing stuff that could result in unpredictably broken behavior ("Why are new pages not loading correctly? Stupid fox, always breaking and needing to be restarted"). Blame Linux' package managers and Linux' liberal way of overwriting files that are in use*. On Windows, a Firefox-specific service takes care of updating the files in between runs, so you can continue using it until you want. * See also the recent introduction of Offline Updates in SystemD, showing how "Linux can do updates on a running system and does not need to be restarted" was always a farce and invisibly broke stuff.