[compsciphd]

simple binaries yes, firefox installs a whole lots of dynamically read stuff that is specific to the version installed (was more of an issue in the xul days, but I think still exists).

i.e. you install a new firefox, the data on disk (i.e. file xyz) is no longer what the firefox version running expects and bad things happen. It could cache everything in memory (I guess) and never have to read anything from disk, after it starts up (relative to it, cache/cookies in local profile shouldn't matter in regards to disk).

so when you update the deb you are replacing file 'xyz' and old xyz no longer exists (as opposed to unlinking and replacing "/usr/bin/firefox" as even though unlinked it will still exist (for paging purposes) until every execution of it terminates and only then will it actually be removed from the file system.

this is where containerization can help on a "multi user" system, if everyone ran firefox in a container, every time they started it fresh, they would get the most up to date copy installed (in a non "multi user" setting, its less important, but can still have some value). (quotes because I mean multiple simultaneous desktop graphical desktop users, which if we're honest, most desktop linux users aren't doing).