The fundamental problem is Mozilla has drifted further out of touch with reality as the funding it receives is not attached to incentivisation of the correct course of action. At this point they look like a social justice movement masquerading as software development organisation, with the net result being a greater appeal to the social justice movement than software developers.
This is just one manifestation of that, and the resulting misallocation of resources. Firefox used to succeed by being better, not by having an ideology. That at one point it had both is a happy accident.
I think there is some truth to your critique, but I'm not sure the conclusion should be that Mozilla should be focusing on software developers, as software developers are not the bulk of its users.
But the original article doesn't just point to a problem with Firefox for developers, but also a problem that makes it hard for developers to create extensions, which can mean a worse experience for Firefox users.
Mozilla's ideology does have the the potential to be focused on user value. Mozilla is, according to its formal and written mission, dedicated to the "open web" (https://www.mozilla.org/en-US/mission/), not to all Good Things. And Mozilla has succeeded in ways because it has consistently held faith in the web, when others have not. Microsoft gave up on browsers and Mozilla didn't, which was Firefox's first big win. Google gets distracted by Dart and (P)NaCL, while Mozilla sticks with Javascript and creates something like ASM.js. Or compare Android and iOS to Firefox OS – Mozilla is really sticking its neck out to support the open web in this case. Firefox OS isn't about social justice, that is a product being created to defend an ideology focused on the open web, and it's a huge allocation of resources by Mozilla.
Extensions actually fit in kind of poorly here, which is perhaps why things are rough. Extensions aren't the open web, and while there's value in Extensions for Firefox-the-product it doesn't have good mission alignment.
All that said, I'd agree that Mozilla, like many mission-led organizations, has a real challenge distinguishing its aspirations from the work that really defines its contribution to the world.
This is just one manifestation of that, and the resulting misallocation of resources. Firefox used to succeed by being better, not by having an ideology. That at one point it had both is a happy accident.