Haven't had any of my 15 extensions break in 20 releases. I don't expect any breakage any time.
The reason for rapid release is that they need to get new features out to load stuff properly like google docs, yes.
The main reason to use firefox IMO is that its as fast or faster than others (and has all the features of course) AND has this: http://www.mozilla.org/en-US/about/manifesto/ to tip things on it's favors.
Also, I'm using Australis and its just fine. I switch back to release sometimes and I don't really notice the difference anymore. I do prefer the australis tabs contrast.
In my mind, the quicker iteration on features and improved code quality that came from rapid releases is well worth breaking a year-old addon, especially one as invasive into the browser as Vimperator.
Addons written with the Add-on SDK (aka Jetpack) are also much more stable across versions, though they're not the browser-transforming beasts that Vimperator is.
I don't really like web browsers as a platform, but I don't think restricting the pace that new features are released is productive.
I switched back to Firefox because it became faster than Chrome (from my user experience, I don't know about tests and whatnot)
Many of these updates are helping them stay fast and haven't resulted in any bloat (yet), so at least from my perspective, great!
Why do people use Firefox? Most users claim extensions. What breaks extensions? Frequent updates. Effectively annulling the most compelling reason to use Firefox.
This is certainly my experience. Pentadactyl, the most compelling reason for me personally to keep using Firefox is more broken than not. Every single update in the last year has broken it and sometimes in non-trivial ways, and stretching my patience to the limit. If I have to abandon Pentadactyl, I really don't have a reason to use Firefox anymore.
UI changes proposed in Australis are not something to look forward to either esp. if you like hiding Firefox UI elements and basically just keeping undecorated minimal window with Pentadactyl.