Mozilla is pretty much pioneering this effort. They've been looking to replace the current extension model for years, as it's not even really an API, it's most

Mozilla is pretty much pioneering this effort. They've been looking to replace the current extension model for years, as it's not even really an API, it's mostly just a way for people to fuck around with Firefox's source code. Powerful, but unwieldy and extensions break all the time, if Mozilla changes anything. Well, and one idea in that process was to not write a new extension API from scratch, but rather use the proven API from Chrome, which therefore will also make the transition much less painful, as a good portion of Chrome extensions will get ported over and we're not starting at square one with the extension ecosystem. Also, they recently introduced multi-process which breaks all extensions anyways, so they're pushing hard to get WebExtensions done, as it means that add-on authors can just port directly to WebExtensions and don't have to port to multi-process-compatible first. And if Chrome continues to eat up market share and extension developers lose interest in Firefox, this will also help out, as the effort of porting from Chrome to Firefox is now often trivial. So, yeah, Firefox 57 will exclusively run WebExtensions, which is Nov 14. Microsoft mostly just saw what Mozilla is doing, figured that they still needed an extension system to get Edge fit for competition and figured that complying to the standard for once would get their extension ecosystem quicker up to speed. As for Google, well, I assume they are not particularly excited about Mozilla embracing and extending their extension API, as well as Microsoft having a slice from their cake, too, but it's not like Chrome was in any danger of competition...