Not very "open", is it?
If anything this kind of thing only hurts the users that know what they are doing (such as someone wanting to install a 3rd party addon or someone wanting to install addons via nix).
If it's totally rogue malware, sure.
The toolbars, on the other hand, were coming from companies like Oracle that operated entirely within the bounds of the law.
* It considered width to include border and padding, when the spec and other browsers did not: https://www.jefftk.com/p/the-revenge-of-the-ie-box-model
* Floated block elements would get their margins doubled, so people would typically use padding instead.
* Its implementation of height was more like min-height.
While I'm sure Firefox and Chrome have open bugs that are older than that, I'd expect them to be much less important than these critical misinterpretations of CSS.
(To give a sense a sense of the scale I think we're talking about, this would be a bug that, say, more than half of web developers have to understand and work around in a typical month)
Youtube using polymer/shadowdomv0 and absolutely destroying performance on Firefox is a an easy example. Google Earth used non-standard APIs.
Otherwise, have a quick search for "non-standard" on MDN, and see how many of those Chrome implements.
I do use firefox but all the auto-update features are off on desktop. On mobile I am/will be stuck with 68 forever, I suppose as well.
In any case, a dev browser can only be relevant if it's representative of end user browsers. The history of browser wars has shown that a superior dev experience can be a decisive factor in winning users, but it can never be more than a supporting role.
These days a far more plausible dev level monetization angle would be Mozilla's unique position as the origin of Rust. Spin out a Rust subsidiary that sells service and perhaps also some premium tooling/SaaS in the open market while doing work on Rust/Servo/FF for a nominal fee that allows both sides to keep the lights on.
But that (just like paid browser dev tools) would be B2B and apparently Mozilla Corp completely fails to see itself as a B2B actor outside of consumer eyeballs deals like the Google deal and the increasingly outbrainy start tab fillers.
But failure to go that way is hardly surprising. Where would the business skillset come from when the entire organizion outside the tech teams has more in common with the great money furnaces of Wikimedia than with anything else?
The only way I can see for that kind of transition to go well would be some form of merger/acquisition with someone already existing in those fields (consultancy and/or tooling). "you take the engineers, we'll be buying services". But the successful ones would be very reluctant to suddenly load themselves with the entirety of Rust. It would probably take someone owner-managed who brought a decent amount of idealism into the equation. Total number of candidates might amount to zero.
Its no wonder they lost marketshare among developers.
If something doesn't work with Firefox, we tell the users to use Chrome.
Even large companies are just optimizing for that one browser:
https://thenextweb.com/dd/2017/11/28/please-build-websites-w...
How many users are going to be willing to put up with stuff not working until they switch?