The idea is to reduce attack surface as much as possible, and disable features like searching/calculating from the address bar, service workers, wasm, prefetch, webgl, redirects, the PDF reader (no files ever just auto-download or open in anything by default), Firefox View, webrtc, network prediction, the social API, reader mode, there's some stuff about certificates, TLS and SSL etc.
There's also a section for preventing fingerprinting, disabling telemetry/reporting/safebrowsing, all the stuff here: https://support.mozilla.org/en-US/kb/how-stop-firefox-making... and to keep changes from being made so no auto-updates, and experiments/normandy/shield studies are disabled.
There are a bunch of guides to harden firefox online, and ours seems to be some combination of several of them. The Tor browser guys do some great work keeping an eye on some of the changes that get made.
In the end basically no active content of any kind is allowed by default. Some sites don't work at all with this stuff turned off or removed and it's a bit too locked down for day to day use but I'm surprised and how often things work well enough to get what you need at least. For example, an article might load just fine, but site navigation will be broken which isn't really an issue if you just wanted to read the article.
Most of the sites that will get you infected just by visiting them require JS, so disabling that alone helps a ton. For something you know is really evil it's often best to just download and analyze the site offline, but for poking around even in highly questionable places it's pretty nice.
Privacy with Firefox out of the box isn't that much better than Chrome’s either, but what sets Firefox apart is that it can be very effectively hardened. At work I deal with all kinds of compromised and malicious websites and nothing beats Firefox once you've got it locked down.
> I’ve used FF since maybe 2008 and in my experience it’s never been worse than it is now in terms of performance, its interface customisability, and its benefits vs its largest competitor.
It's gotten more difficult to customize firefox over the years for sure, but what competitor offers better interface customizability? What can you customize in Chrome that you can't in firefox? What's chrome's equivalent to userChrome.css?