> As we read this, let us reflect that this reasonable and mature position resulted in a ~2% market share
I would disagree.
Many other changes together lead to this outcome.
Many by Mozilla, some not by Mozilla, some you could say Google actively abusing their power to sabotage them.
AFIK the amount of people stopped using Firefox because of XUL isn't to big, you could say a vocal minority.
The problem is compatibility with websites in aspects unrelated to XUL, mostly due to it being way to hard to be bug and feature compatible with chrome and even back then most web applications being written for chrome only.
Two major pain points which make people stop using Firefox would be 1) media support, which partially is the fault of stupid patent issues and partially because programs relaying too much on non standardized aspects 2) CORS, mainly due to Chrome last minute deciding not not implement the standard as standardized and forcing an exception into the standard to still be compliant.
The 2nd point is I think the most common reason I run into cases of web sides not working at all. While I very well do understand that Mozilla devs don't want to make their browser less secure, I don't think they have any other choice and should have done so years ago.
The 1st point is complicated as it involves patents to codecs and the pain of video codec drivers as well as companies like Slack or Teams refusing to even _try_ to work with Firefox when it comes to video calls even through it should work in most cases. This is a major problem and against all best web practices (also a lot of smaller companies which have way less resources have Firefox support for such use cases....).
But IMHO the web needs a cut, it's unacceptable that it's way easier to write a production ready new OS from scratch then a browser which is an essential tool.