You can't have your cake and eat it too. I'm fairly sure that if the Firefox developer team could support XUL with the new internals, they would (just look at the insane backlash for a source of motivation). But they didn't, and there must be a reason. Likely it's due to how integrated XUL is with the internals of Firefox and that couldn't gel with the Rust rewrites.

Oh I totally understand why Mozilla did what they did. It was the logical step to make Chrome addons available on Firefox, abandon the structure XUL imposed, and migrate to Servo-based components. But while they promised a better WebExtension standard (which I defended at the time), they didn't deliver. They needed more time to improve the API and make sure the most used extensions were ready[1]. Developers needed more time too to totally change the architecture of their addons.

And today, they're not ready. And they won't be in six weeks either.

[1] https://arewewebextensionsyet.com/

Edit: [1] answers "Are we WebExtensions yet?" as Yes. I don't agree.

Well they gotta make the cut at some point. If Firefox waits forever then Chrome will just eat up all their users.

Don't listen to the stragglers who want all the old Firefox behaviors supported forever. The wast majority of users don't care about Tree style tabs and vim style keyboard navigation or what XUL even is. They just want something snappy and fast like Chrome. That's how you get users to use your browser. That's how Firefox became popular in the first place. That's how any browser ever became popular. Yes even IE was faster than Netscape at one point, maybe because of nefarious practices by MS, but it was faster, and that's what the users wanted. It's all about the speed.

It's because of the killer combo of increased sandboxing, multi-process architecture and performance constraints.

I think they couldn't keep everything together with the degree of access old plugins had to Firefox internals. So at some point they had to rip off the band aid.

Multiprocess already shipped. One of the reasons extension developers are frustrated is that Mozilla told them to make their XUL extensions multiprocess-compatible and then told them they'd have to throw that work away.

Notice how I carefully avoided your strawman. There were several reasons for their action. I listed what I believe are the 3 main reasons.

They did have the choice to not ship the increased sandboxing, multi-process architecture and performance constraints until three months after the new extension APIs were ready. They didn't because they thought the trade-off wasn't worth it.

They really couldn't. People don't do things until pressured. Look what happened with Python 3. It took 10 years for it to reach Python 2 parity in terms of packages...

The problem with WebExtensions has been Mozilla not providing APIs, not extension developers dragging their feet.

I really do miss the slow and methodical Mozilla that was not out to chase the latest Chrome fad...

Absolutely. I won't be upgrading and I'm already looking for alternatives, as long as crucial parts of my workflow (tree tabs, session manager) have no adequate replacement.

The ugly redesign is also a thorn in my side.

https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta... has a 'Compatible with Firefox 57' badge, and I've been using it with no issues.

I've always been satisfied with the built in session management (restore last session on open) and don't have any feedback on the other.

Not only is the new Tree Style Tabs a step backwards from the old, I'm also not running that addon, but this one: https://addons.mozilla.org/en-US/firefox/addon/tab-tree/ I find it lighter and more responsive.

As for the session manager, I need more features: saving and loading several sessions, etc.

Agreed. The inability to customize button placement and the enforcement of a lowest-common-denominator default placement is painful. Unfortunately, all the other major alternatives are just as bad or worse.

Unsure, but doesn't the 'Customize' option give you exactly the ability to customize button placement?

There are some things you could change that will be going away - such as having separate (and equal sized) forward and back buttons and separating them from the location field; breaking out the stop and reload buttons; changing button appearance; and the like that depended on XUL and are going away with the new "Proton" UI.

Even now adjustments I used to be able to make in seconds by installing an alternative theme now take an hour of wrestling with the Classic Theme Restorer extension to restore and unbreak with each version upgrade.

The session management piece is my big sticking point. I've come to rely on Session Manager, but the developer has already stated it will not be ported because necessary APIs are not available through WebExtensions. Since Chrome has better options (mostly through Session Buddy, which isn't nearly as good as Session Manager, but it'll have to do) I guess I'll be going to Chrome full time.

How's the redesign ugly?

Well, I guess it's subjective, but I find it uglier than the previous design. The obsession with excessive flatness and the deterioration of user experience in favour of "prettier" visuals never sat well with me.

Tree style trends had already been ported

The problem is that electrolysis alone already kept breaking hundreds of addons, so that wouldn't have worked. The only way to prevent breakage, from what I've gathered, would have been to just keep Firefox like it is and not go on with Quantum, which really wouldn't have been great either. And of course, with more users testing WebExtension addons, those new addons hsould mature a lot faster than before.

Oh shit. I had not been following this and I didn't know that DownThemAll wasn't going to work. Now I'm sad :(

At least for the time being, you can use legacy extensions, by enabling the right option [1] in about:config.

[1] extensions.legacy.enabled -> true

You can enable legacy extensions but this is not the same as being able to use them. Many significant and important-to-me legacy extensions (such as FireGestures) are currently non-functional on Firefox Nightly and have been for some time. I can only assume that Mozilla has decided that they don't care about keeping any legacy extensions working other than their own (they have some).