Which extension are you using? I use Mate and found a Gnome extension that was known to be unreliable and indeed did not work...

I agree the restart is a minor issue, as is the menu bar showing by default, as is the bookmarks bar being always/never there (rather than the more sensible Chrome behavior), as is the interruption of my workflow when Firefox decides to check for updates when I start it, as is the relative laggy UI, ... But these issues add up to a worse experience. Worse by enough that despite wanting to avoid Chrome, I came back to it :(

Oxygen KDE[1]. Admittedly, it's not ideal and sometimes lags behind Firefox version changes, but I've been pretty happy with it.

[1]: http://oxygenkde.altervista.org/

There are also some Firefox-specific extensions I can't live without any more, the main one being KeySnail[2] which gives me Emacs-style keyboard shortcuts.

[2]: https://github.com/mooz/keysnail

Are those window controls (close, etc) drawn by kwin or firefox/extension?

They're drawn by the extension. That said, it's almost impossible to tell.

Oh, looks fine to me. I use HTitle[1] on Gnome, but that only hides the titlebar in fullscreen...

[1]: https://addons.mozilla.org/en-us/firefox/addon/htitle/

I see fonts for web browsing in linux are still horrible.

That's actually Hacker News being terrible. Their font declaration is just "font-family: Verdana;" so if you don't have Verdana installed (which just about never is installed by default on Linux), it defaults back to the one mentioned in the browser settings (which tends to be a serif font).

Hacker News could've made their font declaration have a fallback (like "font-family: Verdana, sans-serif;") and things would look way better by default.

As for me, I solved it by editing userContent.css and adding a font-declaration for Hacker News myself. (or you could use something like the Stylish addon)

All in all, I'd say it's the fault of website creators.