I would also like customizability to stop being trimmed away, and ideally for some of the things that users currently accomplish with userchrome hacks to be supported directly one way or another.

The number 1 thing I want to see from Firefox is fist-class support for tree tabs. Using an extension and hacking on userChrome to hide the tab bar is getting tiring, especially with the barriers Mozilla have erected to make user styles more difficult to enable.

… and say no to Web Extension manifest v3, or at least make long-lived background pages an option instead of short-lived service workers.

Canonical is working on firefox snap performance: https://snapcraft.io/blog/improving-firefox-snap-performance...

That said, I've switched to using the firefox tarball. Start up performance isn't the only issue. Snap firefox doesn't seem to respect my dconf settings and I experienced some strange hangups that forced me to kill the browser.

Self-hosting Pocket could be phenomenal. I would gladly donate what a subscription costs if I could simply have it myself and improve it.

Have a look at Wallabag. You can self host it, it has browser and phone plugins, and works quite well.