And sigh, yet something else I'll have to disable (or remember to disable) on new installs and upgrades. I wonder if non-savvy people will be able to figure out how to disable it so it doesn't interrupt their time on the internet.
It is much easier to flip a preference. If you wanted to get sophisticated, you could create an addon that disables all of the preferences you dislike.
That's a good idea. Another option is to patch the code to remove the new features, unless Mozilla has taken steps to deter unbundling.
If there's a big enough market, perhaps some company will backport the security fixes. How much are you willing to pay? Another option is that you could volunteer and start the fork.
Of course, there's that other problem where developers are only going to support the latest browser.