The rest of us are working on stuff like: Fission (process-per-tab, roughly). Pageload performance. Memory usage. Printing. Moving stuff to the GPU. The never-ending treadmill of new web APIs. Porting to new platforms. Fixing bugs.
As for extensions: there is no intention to regain 100% of the power of XUL extensions. Never has been. Many extensions used to get the source code of an internal function, search and replace to update the code, and reinstall it. We're not going to return to that level of power. (Yes, XUL extensions don't have to do it this way; I'm using an extreme example to illustrate the former power.)
But that's not to say that Mozilla doesn't want to increase the power of WebExtensions. I'm not really privy to the decisionmaking there, but look -- we just laid off 250 people. I hope that's adequate proof that we have to make some hard resourcing decisions. And most of the "why don't you just..." requests are not as trivial as they seem, even when the requester's exact use case would be fine. Any extension API that allows deceiving the user or subverting their intentions must be handled carefully to maintain the security of users, and people are pretty darn good at finding unexpected ways to exploit seemingly innocuous capabilities.
I'm not going to debate the history or correctness of all the decisions that bother people. I agree with some, disagree with some, and aligning my views with yours or anyone else's isn't going to have much of an effect on anything. We've screwed up. I'm sure we haven't properly admitted to some screwups. People's experiences and workflows are different, and I know I'm always surprised to discover that 99% of users are not seeing something that bothers me. My intuition of what is or isn't important is not as good as I expect it to be.
When you write about Firefox, you seem to write about various side projects like Firefox-Send, or Pocket that barely add any value to the actual users. Users don't want them (or even ask for them) - yet the teams are very happy to start and make some half-baked greenfield product [after all Firefox Send is a project failure - couldn't stop malware + nobody used it].
This might be a blasphemy, but IMHO even creating Rust is a bad side project. [maybe good for the world, but definitely bad for Firefox - resources are removed from browser development team]
2) Your comment makes me lose hope. Users are very clear: they want extensions. Yet you act as if XUL extensions is the only way to make extensions. Ever.
Do you mean that the whole Firefox team (probably 1000 people) cannot invent a new and secure extension model that allows to adjust the browser per user needs? I get it - creating it is hard. But it is literally your job. When you are a Firefox programmer you should work on the core product. Not on some easy side-project.
From outside perspective it looks that nobody wanted to deal with the difficult problems. Management just let the developers to ignore them and create new, sexy, greenfield side projects.
Another user pointed out that the new extension model does not even allow to remap keys immediately (personally haven't tested it) - most computer games allow this, so don't act as if this was some impossible programming challenge.
3) When you release unfinished, buggy products you alienate your user base. Most users understand that software can have bugs. But why does Mozilla constantly release things that break the users' work-flow? From user perspective it feels like change for the sake of change. You start to wonder what feature will the new patch remove or break.
At the same time your competitor is reliable. They can change the inside, but they don't touch the outside: the user interface stays constant.
4) The problems with extension signatures is a complete fiasco of the Firefox project management. It shows that inside you have zero control over the process - no tools to track workflow, critical tasks and what needs to be done? (are you still stuck on mailing lists?)
This one is 100% on Mozilla and please dont come up with some excuses, because they sounds pathetic.
I repeat: from user perspective it looks like a very small part of Mozilla deals with the actual Firefox and everyone deals with other, random stuff. Some gatekeeper had one job: sign the extensions once per year - and failed.
Also when you sign the extensions using some strange side channels, you sound like hypocrites when you claim that Firefox does not spy on its users.
5) Instead of focusing on core product, old bugs, user requests - the teams prefer to make "sexy" greenfield projects.
Proving proper Firefox installers to companies, means higher market share and consequently more money from Google. It's like someone at Mozilla was sabotaging the company from inside. Sorry, your argument that providing installers cost money is just absurd. Why you dont use the same argument about Firefox-Send, Pocket, or 20 other side-projects that are leeching development time/money from the actual browser? Browser is the one that earns money, but it is stripped from resources that are used for "adventures" of developers. Like the Firefox Send "adventure" that was a failed idea from the start, failed at the end. That money could have been spent working on your core product.
Also, when we speak about installers for corporate multiple people claim that inside Mozilla, there was a stubborn idiot that didnt want it to happen. It is a complete failure of management who didnt identify that person and fire on the spot.