For me it's a long list of missing features (using Mac OS).

* No Chromecast support

* Missing pinch to zoom support (smooth as butter in Chrome). Possible via Add-on "Multi-touch zoom" [1] in Fx but it's very sluggish.

* Youtube is much slower, also some features don't work there, like being able to scroll down when having a fullscreen video open. Can be fixed by installing "Classic Youtube" [2]

* Fullscreen is broken for me, I was not able to hide the address bar and tab bar. I had to install "Fullscreen Plus"

* Some extensions don't work as good, like Dark Reader [3]

* No good alternatives for some Chrome extensions, like "The Great Suspender" [4] or "Vanilla Cookie Manager" (Cookie AutoDelete for Firefox)

* Pdfium performs much better than Firefox' solution. Unfortunately Mozilla abandoned Project Mortar [5].

I could go on with the list. Don't get me wrong, I will still stick to Firefox even though Google Chrome is the superior product performance- and feature-wise.

[1] https://github.com/haxiomic

[2] https://addons.mozilla.org/en-US/firefox/addon/youtube-class...

[3] https://chrome.google.com/webstore/detail/dark-reader/eimadp...

[4] https://chrome.google.com/webstore/detail/the-great-suspende...

[5] https://wiki.mozilla.org/Mortar_Project

Take for instance the LastPass plugin: You can't just copy the current site's user name/password* with just 4 mouse clicks like you can with Chrome's extension, you need to edit the site entry (4 clicks), click show password, select the password, Ctrl+C. If you need to do this for both your user name and password, it gets a lot worse than in Chrome (which remembers your position).

And don't get me started on the really bad font rendering. Firefox is still behind Edge and Chrome, hell, even IE (at least on Windows).

*: something you need to do more often in Firefox because the extension's autofill feature is even finicky than in the Chrome extension.