Over the past dozen+ years (going back to FF 1.something, when still on WinDoze), I have installed many add-ons. These days, around 30 survive, and while I'm sure some can be removed (for example, don't really *use* ColorZilla anymore), with nearly 90,000 users, my favorite by far, and the one most indispensable to daily use, is Tab Groups.
As a "Legacy" add-on, it will, due to Mozilla's mandate, not survive the upgrade to v57.
It's more than just a bookmark or history manager, and there is nothing like the functionality it provides in the new FF. Containers don't cut it; don't want a huge vertical "Tree" view. Want that familiar icon that helps me organize my tabs into logical collections, letting me switch to a different group, or being able to right-click on a tab and move it to another group.
Some months ago its author announced he would not be converting it to WebExtensions, and has released its source code to the wilds of GitHub. I, alas, do not have the free time required to dig in and figure out how to perform the conversion.
I currently have 35 groups, one just for /. That group has about a dozen things in it, related to exploration of various stories I've read on the site. It's very handy being able to organize my surfing in that manner. The groups do get pruned from time to time, if after a bit I fail to follow up on some page that's been saved to a particular group, or when cleaning out base search queries.
(There are "Containers" in the modern FF world, and one very nice thing about them is keeping cookies, etc. separate. That's a *good* idea. Tab Groups does not do that, but I hope its successor does. Unfortunately, any WebExtensions add-on I've seen which employ them falls far short in doing what Tab Groups can.)
Am in general pretty loyal to my technology, so while they're on the machine, don't use Chrome, Vivaldi, Opera, etc. Am on Mac these days, so M$ browsers are out (even in the WinDoze days, they sucked. Anyone remember IE's skinned cousin Maxthon, which sucked ever-so-slightly less?)
Thus, FF it is, and until something so radically better comes along that I needs must re-evaluate my choice of browser (as did it, rising from the ashes of Netscape, which I had used since v2 back in the 90's), FF it shall continue to be.
I totally understand the developer's recalcitrance to re-write his entire app. I also totally understand Mozilla wanting to push their browser into the future, but feel they are falling into the all-to familiar trap of not only desiring that I use their code, but deciding for me how I should use it, as well.
There is no technical reason why Mozilla could not allow the performance hit of having legacy extensions remain functional. Sure, it will add bloat to the program (think multiple sets of libraries being needed to accommodate XUL or XPCOM as well as WebExtensions), but Mozilla should give developers more time than they have, to catch up with the new way of doing things, or let new faces take up the old code and convert it, rather than simply throwing years of good work down the drain.
The whole idea of an add-on is that a third parties may add functionality to a program which the original authors have not coded in to the core. (Just imagine how painful using *nix would be, without bash scripts.) While WebExtensions is much more secure (it limits what the add-on can "do" in modifying the core code's behavior), many, many popular extensions have already been written - and vetted - by Mozilla, using the older XUL technology.
While I agree that moving forward, new extensions should be forced to use this more secure way of doing things, Mozilla should also recognize the contribution of earlier add-on authors and allow older extensions to run if the end user desires it .
Thus, until something comes along which can provide the functionality of Tab Groups, and is written using WebExtensions, I have sadly been forced to turn off FF upgrade notifications.