I’m a long time Firefox user. Because Firefox is far from perfect, I installed a bunch of extremely useful addons fixing all the annoying things. Firefox is like The Edler Scrolls of the web browsers. But that’s ok. That’s what I loved about Firefox. You could fix everything. Even your developers saw it this way. Sometimes when someone reported a glitch or an annoyance, you just told us “don’t worry. Someone will make an addon for this. Bug closed, won’t fix.” Example? See here: https://bugzilla.mozilla.org/show_bug.cgi?id=942461#c8
You see that addons are very important to the Firefox user experience. Even your developers relied on addons and would have made different decisions if there were no addons. Firefox has evolved around the addons and depends on them as much as the addons depend on Firefox. If you kill the one, this will kill the other as well.
Now Mozilla is sabotaging Firefox. You make drastic changes to the addon API such that many of my favorite addon developers already announced that their addon will no longer work (like FindBar Tweak - a must-have!).
Also there are many legacy addons that are crucial for me (like “Password Hasher”. It was abandoned because the developer switched to Chrome. What an irony. But if it stopped working, I won’t be able to get my passwords for online shops etc. any more. This would be a huge catastrophe)
As far as I see, Firefox 57 will be doomsday. Then, years of customizing, installing and fiddling with addons will be lost. I will have lost access to all important online shops and have to start all over again.
What’s the alternative? I don’t know. I know there are good reasons to switch to WebExtentions. But for power-users like me, the whole situation is extremely frustrating. Backward-compatibility is the most important thing in my option. If you can avoid breaking everything, avoid it! Just because it’s called Firefox doesn’t mean you have to set everything on fire once a while.