It's exactly the opposite of this. Have you read through the FF update notes? The web standards are a moving goalpost - constantly evolving.
On top of that they've been slowly landing incremental improvements to Electrolysis that have allowed them to turn it on for more and more users without big risks to stability. You can't do that for a browser with "big bang" releases every year.
In the last years I think it's been shown that the only model that works for browsers is "evergreen".
If you're intentionally running very old versions which by now have published exploits, then I can see that these are not things you would consider important.
I'm now running the latest version of FF, but still able to use my plugin.
source: http://www.ghacks.net/2008/07/09/change-the-session-store-in...
I'm holding out hope that whoever's responsible for Firefox abandoning "good, light, cross-platform web browser" as its differentiator stays away from Servo, at least for a while. That's a niche that's unserved currently, AFAIK.
Firefox really started to annoy me with its constant and needless updates a few months back; the tipping point being breaking almost all legacy extensions (in 46, I believe). This totally broke the Zend Debugger extension, the only way forward would be to totally change my development environment. I'm 38 and now, and apparently well beyond the days when the "new and shiny" hold value. These days I just want stability and reliability.
Firefox keeps charging forward and, as far as I can tell, has brought nothing to the table except new security issues and breaking that which once worked.
I haven't updated since 41 and you know what, it's nearly perfect. It's fast, does what I need it to do, and just plain old works.
Firefox appears to have become a perfect example of developing for the sake of.