Mobile's still broken. Which is a browser Mozilla has apparently abandoned the userbase of in favor of focusing all their mobile effort on some silly pointless separate browser that caters to millenials more somehow or some other nonsense, instead of just working on the browser they already have. Really fed up with Mozilla at this point and considering switching to a fork or something.
I still hold that the multithreaded performance benefit was nowhere near worth wiping away so many of hours of developer time and ripping so many good extensions out of users' hands with no replacement for so much of the lost functionality.
As a regular user, it was worth it. Firefox was honestly pretty shit on (at least on Windows) pre-e10s. Before, I had to kill firefox every few days because CPU usage would climb for no reason. Since then, the only restarts I do are for updates.
As a browser, it works much better, and as an extension developer as well, I'm glad I can write one extension that works in most browsers now.
Yeah, it sucks they removed the level of customization they used to have, but overall that changes are welcome.
That said, this whole thing is a huge weakness to me: having some organization decide what I can put onto my own computer is a frustrating tradeoff, but now it's gone from a nuisance to a real fucking problem.
I'm not an imbecile. I can manage my own browser plugins. Give me a switch to install XPIs without having some authority sign them, or at least verify once on install and piss off after that. I don't like my devices phoning home every 30s to make sure I'm "safe."
xpinstall.signatures.required in about:config is the switch you are asking for. Though, it does not allow one time checks at install, but a choice of regular checks, or no checks at all.
It's not just performance that was the issue. XPCOM gives you access to basically the entirety of the browser internals. There's no designed API, the implementation is the API. Which means that you can't change the browser internals, unless you're willing to break addons, or you perform an audit to figure out which plugins will be broken and you get them to update first.
Basically all refactors took months and months and months because of this. There was no way to address the accumulating technical debt.
