Also take into account how many Chrome extensions are ported over now that it's easy. For example, the only extension I use that won't get ported over is VimFx, but Vimium has now become available which does exactly the things I used VimFx for.
Unfortunately 75%+ of the most used extensions either arnt ported, wont be ported, or cant be ported.
The situation is pretty grim, mozilla really needs to delay this move, web extensions arnt capable of supporting the functionality lots of these addons require, and its not like XUL is even going away in the near/midterm, this is purely an administrative decision.
If they plow forward anyways its going to really, really hurt firefox.
That said, saying it's an administrative decision really misses the point. XUL support hampers performance, slows down Firefox development, and includes security issues. Those are serious reasons for moving away from them. They might not weigh up to the benefits to you, but you can't deny that they're there.
Such as the fact there is no way to enumerate what sites have html5 local storage stuff stored, thus no way to control or clear it on a per site basis, and they are not interested in changing this, rendering an addon like what self destructing cookies does imposisble...
https://bugzilla.mozilla.org/show_bug.cgi?id=1329745
Or the ability for extensions to construct permission sandboxing
https://bugzilla.mozilla.org/show_bug.cgi?id=1353468
There is objectively a LOT of functionality that is just plain never going to be implemented, thats far from the fault of the extension authors.
Many have been trying to work with mozilla to push in the functionality they need, but if they just get disregaurded and ignored what are they supposed to do exactly?
Don't try to place the blame for this on the extension authors.
When I was porting my extension, my big worry was "is there going to be something I need to do that doesn't work?" But there were no show-stopper bugs.