Jesus from the rest of the comments, you'd think Mozilla was removing the XUL interface just to spite people. It's being removed because it's incompatible with making Firefox multi-process and sandboxed. Firefox is literally the only major browser that hasn't moved to a multi-process and sandboxed model yet, and it shows hard in performance and in security vulnerabilities. I'm very excited to see Firefox finally catch up.
The "XUL Interface" is largely a misnomer. Firefox has had extension interfaces that are compatible with multiprocess firefox for years. Yes, you can break compatibility, but if you do it right it will keep working. Also the browser will tell the extension that it's not allowed to do that so the extension developer will know to fix their extension.
Don't get me wrong. I love the webextension idea. But it doesn't replace the ability to have extensions with the same power as the browser itself. I would love for Mozilla to promise compatibility for webextensions and say "you are on your own" for traditional extensions. They can mark them as bad in the store, even refuse to fully review them so that I know that they are why my browser is broken. But sometimes that power is incredibly useful, and I would hate to see that taken away.
Like many others the only reason I use firefox over chrome is because of a couple of extensions and if they were gone I would switch because internal sites at my work tend to break less and internal extensions generally only support chrome.
It would except the user interface will be stripped so bare you can't actually have more than a dozen or so pages open at once! I know there are many people in the same position as me. They open a lot of tabs. When Google killed off vertical side tabs (which were only ever a hidden option you had to enable manually anyway), they went back to Firefox. And now Firefox is taking the vertical side tabs out in the street and shooting them too.
If Firefox included vertical side tabs and committed to supporting many-tabbed browsing as it went pseudo-Chrome-mode, I imagine many fewer people would be irritated.
