https://github.com/runvnc/tersenet
If you glance at that then you may see that I am for the idea of leaner alternatives to the current web platform.
But in the context of the existing web API which has just about everything and the whole kitchen sink in it (hundreds of sub-APIs), I do not think it will really help anyone at this point just just stop adding features, especially major ones.
The web is basically an overlay operating system and has been for many years.
Not OP but I think you are misunderstanding the interaction as a whole here. The Chromium team made a proposal, then the Chromium team asked the Firefox team for a position on the proposal. Whether or not the Firefox team or anyone on the Firefox team has any goals around AI or whatever, this response was simply "We do not like this proposal for these reasons..."
How to fix those issues really isn't the Firefox team's job and also wasn't part of the question asked by the Chromium team.
There are a lot of people reading his position. One or two additional clarifying sentences to spell it out for people skimming is not such an unreasonable ask.