It took me (years?) to upgrade from Firefox 3.x to 6, but then only a week to upgrade to 7.
6 to 7 was seamless and added handy features - so I am changing my mind on the rapid release schedule as long as extensions aren't broken (often).
(7 is in release candidate build 2 already, give it a shot - it feels insanely fast)
It's just numbers, they dont mean much.
Now I just mentally shift them to the right so they are minor version numbers in my head:
ie. 1.6 1.7 1.8 etc. and then I can deal with it.
But 3.x to 6 is a big jump. Took me an entire weekend of fiddling, tweaking and lots of googling and research to replace extensions and get everything just right.
I think they're working on the new extension framework to allow reboot-less installs, etc, but their extension developers aren't adopting them quickly enough.