Maybe I can shed a little light on the not making any sense behavior of users:
I am one of those users but with one exception: I stick around. Have been around long before Mozilla firefox was born. I probably was adopting the browser in its early infancy. I came from netscape. Prior to that all there was for me was Cello and a little later Mosaic. When netscape came along.
Firefox was the ONLY alternative for me especially when microsoft bullied their Internet Explorer down out throats.
Enough of my parallel history - The reason many are leaving mozilla is, the first instinct of humans is, to fear change. Even the ones that face change, do so hesitantly.
Now, take a scenario where you are the user, you have your browser just ever so perfect tweaked over time. One morning you open your browser up, and [hyperthetically] you dont see your bookmarks, or dials. Next step you try to think, if it was you yourself that did something wrong the night before, having worked too many hours and were tired.
You then check the addons: and suddenly a entire slew of your addons have a loud yellow highlighter fever: Legacy. So, next step is you hurry to the addon page to see if others also have a problem.
And there you read with horror, that your daily usage of this addon may be killed off alltogether.
Panic, because you have to have your bookmarks or dials like the were yesterday.
Then you go to the developers forum for that addon, to find out [still hoping that this all is just a mistake] if and how fast they can restore or roll back so you have your addon working again.
That is then the moment when reality hits: you find out that there is an actual date set, when the addon world goes dark for you. If you are smart, you google and read about it as much as you can find out. You also know by then that your use of that addon may no longer be possible, because the author who spent the last 10 years refining this addon, simply cannot promise anything right now.
By now I hope to assume that you may start to get it what your average consumer feels: abandonement. He/she was not told of the looming danger. He/she read somewhere that firefox will only keep ca 3000 extensions and scapping the rest of the over 20 thousand addons.
Also, somewhere in the google results there was a sentence that Mozilla rested a little too long on its laurels from the early success days and also catered too much to coders.
Now, Mozilla will make a swift move and throw the baby out with the bathwater.
Your user feels he or she is completely ignored in that entire process. Yes, yes, there is loud declaration about how much faster, lighter etc. the new version will be. That would be welcome, yes, but will be instantly forgotten, when the user can no longer use or even see his/her bookmarks or dials. No speed in the world can balance this out.
The search for the next cheese corner for browser is their target. They will pick the next best thing, because they feel abandoned.
No, I am NOT a abandoned feeling user. I have long learned to not only always back up what is important for my online well being, but also know that there is always a negotiable solution.
It’s just, your average user does not calm down fast enough to start thinking that a change may not be so bad. - That is where you see them exit rather fast.
It is the “Who moved my Cheese” syndrome. If you are not familiar with it you can watch it here.
goo.gl/YT4P3x .
If all this sound patronizing - I sincerely apologize. This is not my intention. I rather meant for depicting a situation that most users find themselves struggling with. And these days it seems anger flares up so quickly.
I for my part, have of course also encountered some changes, but my solution was and is: report to the developer with an accurate account of cause and effect of the addon displaying with possible scenarios. That enables the developer to find a temporary workaround rather quickly.
I have seen in one developers forum tempers flare instantly, even name calling etc. - those are the ones that forgot within 1 second that this very addon has worked over the past ten years flawlessly, with the developer using his free time and countless hours to provide a free addon for users.
This particular developer has an addon that is not so simple ported. It has to be written anew from scratch. Am not sure if he can manage in time. I hope they can.
I of course will remain with using Firefox. But I have more Legacy highlights than clear ones. So, for my work, I will need to use Waterfox for a while. I have created literally a carbon copy of my current Firefox setup. It’s not my preference, but I too need an interim to see what all I will “not” have anymore in Firefox 57. Then go from there. Since a so called “alternative” for this particular addon does not exist. Not even close. Plus, I would be forced to manually create over 2700 entries - and I mean really “manually”. Just not a time saver which ever way one looks at it.
I would never switch to chrome or any other - that said. I do hope that Mozilla matures in to a less frenzy schedule one of these days. 
