IMO Gutting your developer base is seldom good.

The web developer base is far more interesting and relevant to Mozilla's mission than the extension developer base, and Quantum is the only shot Mozilla has at regaining mindshare there. Looks like it's working, though.

Here's the thing though. Developers understand the technical differences between Chrome and Firefox. To 95% of general users they don't see why they are different. They look about the same and hey, I get Chrome offered to me every time I visit Google so why not use that? The average user doesn't see the difference that gives them any reason to want Firefox. Now the extensions base will be essentially the same, stripping away those that were truly unique. The extensive UI control is disappearing. Firefox (IMO) is slowly stripping away the things that make it unique to 90% of its user base who just see the UI.

Tgis is not true The extension base is not the same. Firefox 57 will have tree style tabs, for example.

What developer base? 99% of new addons are Chrome first/only (like uBlock, uMatrix). And most of the legacy addons were abandoned long ago.

I use several legacy addons and most of those haven't been abandoned? They are about to be though.

Indeed. Just look at PocketPC vs Mobile/Phone/whatever.

Of all companies one would have thought Microsoft understood the value of backwards compatibility...