Do you often find all your preferences, history and bookmarks are deleted when upgrading Firefox?
No, but I do find a bunch of stuff labelled "no longer supported" or plain ignored. Just because there's a value in the prefs.js file doesn't require the browser to pay any heed to it.
Not upgrading a browser must be one of the most idiotic things to do, given each new release - from any of the big three browsers - is awash with security updates.
Blindly upgrading any piece of software just because there's a new one is idiotic. New releases may have security fixes that I care about, but they may also have performance issues, bugs, and functional regressions that I care about more. I've lost track of the number of times I've upgraded FF or Thunderbird, only to have a "why the fsck did they do that" moment, followed by a downgrade & restore of all configuration. I'm generally several versions of FF behind the bleeding edge.