in the past, this was because firefox was very dynamic loadable (i.e. not keeping everything it needed in memory) so when you updated, you were overwriting data that it depended on.

I argued that this was a positive reason for snap (updating the image behind the scenes and only getting the new version when you restart). However, snap designers (in my not so humble opinion) missed the boat (to put it politely), as one can only upgrade a snap image when the application is not running. So one has to exit, update, wait, restart. This is missing one of the primary benefits of container based delivery for desktop applications.