What was the reason that made the hypothetical 90% go for Firefox in the first place? This seems like the kind of feature one would expect when installing Firefox.

Mobile addons, privacy respecting defaults, not wanting you to be logged into a Google account all the time?

I don't think "makes imgur gifs stop playing" is going to attract more users than it drives away. By all means have that as an option, but it'd be a terrible default experience.

Mozilla demonstrated they don't care about retaining old users when they broke all the extensions in 57.

If you made videos stop playing, users could switch to Chrome as a fix when their gifs stopped playing.

Firefox 57 changed to using a more limited extension system like Chrome already had, so it's not like users could jump ship to Chrome to get back Firefox 56's more powerful extensions.

You may not have liked or agreed with that change, but I don't think it actually drove people to alternative browsers. If anything they might have said "Well, I have to going extension browsing anyway to find new ones that work, maybe I'll take a look at the options in Chrome too."

If I hadn't already been running Pale Moon on Windows, it would have made me switch to it.

And now a short time later they basically all work again but its better than ever and less work to maintain your extension.