UPDATE July 2023: I just did some testing, in case you were wondering just how bad this browser is in terms of privacy. So, it sends 112 requests to various moz domains during your first run of it. But that's not enough for them. They have this crap called Firefox Glean that reports almost every interaction you have with Firefox to Mozilla, with a browser session ID, unique user ID, precise timestamp and various system information included. These requests happen anytime you visit a menu (Addons, Passwords, Settings, etc), change a preference, open a new tab, click through one of the four prompts that appear when you first run the browser, or do literally anything else. And every time you turn off Firefox, it sends a giant request to Mozilla containing information about pretty much the entire state of your browser at the moment of closure. It is quite eerie, if you watch it from the inside. So go whip out mitmproxy right now (it is easy) and see for yourself. This alone should be reason enough to trash this browser, but there is more. Firefox contains links to several big tech sites (that it pretends to fight) inside their New Tab page.
They also kept the Directory Tiles (sponsored pages) in there, that I thought they've killed. And of course clicking either of those is tracked. The UI reveals very few options for configuration, as expected. And about:config requires two fucking confirmations to enter, and isn't intuitive at all, anyway. I could go on and on about specific issues, but to understand Firefox in more depth, you have to dig into the history of it and its corporate creator - Mozilla. You will learn why all the anti-privacy stuff is in there, why you can't install addons without Mozilla's approval, why XUL addons got deprecated, why Firefox ignores user choice in terms of proxy settings, and many other juicy details. Knowing the context behind what's going on will provide you with a lot more understanding than just burying you with more specific issues, so go read the article.
But I have to mention some more relevant points for potential users of this browser. It uses ~260MB RAM with one empty tab; significantly more than that of many Chrome-based browsers and more than twice than Pale Moon. It requires GTK3, like all its forks. And those are all a lot better than the original, since they at least remove some / most / all of the spying. So, Firefox itself is completely useless. What about the others? They are all dependent on Mozilla. And consider that in August 2020, Mozilla has ceased caring about technology, but instead fully began focusing on social issues - "From combatting a lethal virus and battling systemic racism". As if it wasn't already obvious earlier, they have now thrown out 250 people mostly working on technical stuff such as their rendering engine or browser security. I suspect this is preparation for ceding control of the web browsing ecosystem to Google soon (as predicted in Mozilla - Devil Incarnate, they were always controlled opposition). I doubt any Firefox-based browsers will survive this apocalypse, to be honest. Just know what you're signing up for, while you read the below.