There are some people (like me) who use firefox because of addons that can't be supported on other browsers due to the lack of API support, so it's a bit disappointing if they're going to pull the rug from under those addons. If they do, they might see people leave for another browser.
For reference, the addon in question for me is Tabs Mix Plus.
But the core APIs are the same as Chrome's. The implementation might not yet always be perfect, but the APIs themselves are stable. So, for the vast majority of add-on developers, the experience will already be much less burdensome. Or it will be in the future.
Yes, some add-ons will not happen or might only be possible long after the release of Firefox 57. But they're not going back on that plan. They haven't just forbidden these extensions for funsies. They've very much already used the opportunity to rip out tons of legacy code and refactor things that they could have never refactored before without breaking tons of add-ons each time. At this point, there's very few legacy extensions left that still function in Firefox Nightly, even if you tell it to load them.
Not to mention that this has helped them to speed Firefox up by a significant factor. All the performance improvements since Firefox 48 would not have been made without this deprecation on the horizon.
And considering that a significant chunk of Firefox users use no extension or just the obligatory ad blocker (or any set of extensions that's going to be easily replaceable with WebExtensions), this is also very much the decision that has to be made when asking yourself what's best for users.
As for people possibly leaving Firefox when their favorite extensions stop working, yes, that's possible, but Firefox's market share has been growing again since Firefox 48, and there's also just the fact that Firefox is still going to be the browser with the most powerful extension API.
You're not getting more extensions by switching. You'd have to be fine with even less extensions in exchange for maybe a particular feature that you like in another browser.
You have basicly told us all "your problems dont exist, dont matter, or we dont care, deal with it"
I'm done with firefox post 56.
I have no problem with you being sad about these add-ons going away. I do have a problem with you acting like what Mozilla does is completely unreasonable and objectively bad for users.
And due to jilted lover syndrome, I feel compelled to post in threads like this one saying so. Heh.