In a few years the all new HTML Firefox will come out. My bet is that it will suck. It will lack a TON of features that the existing Firefox has, but hey, it's all HTML! And you won't be able to stick on the old one, because within a week or two some critical security flaw will be discovered and eventually (like six weeks later) they'll stop supporting those for old Firefox.
Initially the HTML Firefox will suck. When you take an app that's been worked on for 15 or so years and then replace it's UI you're going to lose a TON of features. They'll slowly reintroduce some of the most popular features (hamburger menu will be priority #1!) but there will be a TON that they will not reintroduce. Why? Because when they were first introduced a decade ago it was a cool idea someone had, and no one knew how popular it would be, so heck, why not implement it. But now they know that only 10 million or even 1 million people use that feature, and they're only interested in 100 million user features! If Google Chrome doesn't have it, it must not be important!
As much as people complain about XUL not looking native, wait for HTML Firefox, it will take them forever to get where XUL was years ago.
They can't just kill XUL for Firefox though, they have to burn down the XUL ecosystem first so they're just releasing a new Firefox, nothing to see here.
1. They try to kill xulrunner as a project separate from Firefox. They try to move everyone to firefox -app.
2. They stop releasing binaries for xulrunner.
3. They deprecate XUL extensions.
4. They distance themselves from Thunderbird. They say it's better for Thunderbird. Yeah right! Thunderbird is built on XUL, it's not going to be rewritten in HTML any time soon, definitely not by volunteers. It's not going to be able to maintain XUL either, and when Mozilla stops supporting XUL for Firefox a few years after deprecating XUL extensions then Thunderbird will be screwed, but hey, it's not our project! We abandoned it years ago!
So when the crappy HTML Firefox shows up, with way less features than the Firefox of today, remember that this (Thunderbird) was one of the things given up to have it.
But hey, donate to Mozilla! $5, $15, $25, anything helps. Because we already make hundreds of millions of dollars and we do whatever is shiny and new, screw the "community" of existing stuff. We're fighting for an open web! (where you can use Gmail for email)