Add-ons like NoScript will be severely limited in their feature set as well. Say byebye to Greasemonkey and hello to Tampermonkey, with it’s limitations. Want that add-on that lets you change the new tab page for something else or enhances that page? Maybe it will be available, maybe not, depending on if and when mozilla kindly provides WebExtensions APIs for such things. And of course, depending on if there will be an author creating this entirely new add-on from scratch.

What this also means: Almost all your existing add-ons will be broken, entirely, save for some Add-on SDK add-ons, namely those that don’t do anything fancy. Sure, even today, lots of add-ons break, and some add-ons will not get updated when they do and there are no suitable replacements. However, with this change, almost every add-on will be completely broken and in need of major updating by the extension authors. Good luck with that.

It is safe to say, that Firefox will not be Firefox anymore as far as extensions go, but instead will become yet another Chrome-clone.