One advantage of these niche browsers is that they don't spy on you, but what I've learned from trying probably all of them is that, in the end, addons are essential - especially uMatrix is irreplaceable. So, for a day-to-day browser, you have only two options: Firefox based and Chrome based. Since they all support the same addons (with slight exception in Pale Moon), we will have to use some other criteria to judge these browsers. Let's analyze them one by one:
There is a long history of anti-user decisions with this one, such as removing many configuration options (or shoving them into about:config, for example https://bugzilla.mozilla.org/show_bug.cgi?id=873709 (archive)), being given millions for including anti-privacy search engines - Google and Yahoo - as the defaults (while pretending to be funded by donations), claiming to care about your privacy (archive) -
Individuals’ security and privacy on the Internet are fundamental and must not be treated as optional. when it HEAVILY spies on you (archive), putting advertisements into the New Tab page (archive), ceasing support for certain addons, forcing all addons to be approved by Mozilla (archive), and many others. The question is - do you want to rely on software from a corporation that does these? For me the answer is no. So what are the alternatives?
Firefox fork from the Free Software Foundation. While the older versions still had some of the spyware found in FF, 60.2 removed all of it, as far as I can see. It also includes a third-party request blocker preinstalled; but despite it having lots of configurations options, the UI is much more basic and not as useful as uMatrix' switchboard. It's nice, though, that there is a browser out there that has even bothered including something like that by default - and the newbies might appreciate the simple UI more. Keep in mind this version has not been officially released yet.
Again, a browser pretending to care about your privacy (archive) -
We’re obsessed with protecting your privacy. That’s why we’ve made Waterfox Private Browsing more powerful than the others., when in fact Waterfox does nothing whatsoever to protect it and actually spies on you almost as much as Firefox (archive) (it made 109 unsolicited requests upon my run of it). The
more powerful private browsing mode is a sham as well - anyone caring about their privacy will not rely on this but install essential privacy addons, so his deceptive claims are designed to lure in newbies only. Therefore this slimy dev and his "creation" can be safely ignored.
A slightly different fork of FF, since it's of an older version, before UI change. Because of that, many addons designed for new versions will not work (but old XUL addons, deprecated by Mozilla, will). Though the old UI is nice - and the dev seems to respect cutomizability and user choice slightly more than vanilla Firefox - Pale Moon also spies on you (archive) (for example, its start page, to which it will automatically connect to - makes requests to Facebook, Google and many other trackers). Nothing to see here either, then.
IceCat is the only Firefox fork worth bothering with, as we can see. Now let's see if the other group of addon-rich browsers has something better to offer:
This browser has made waves thanks to its built-in privacy protections - such as AdBlock, HTTPS everywhere and script blocking - but in the end, they are outclassed by uMatrix, and installing Chrome extensions is only possible through compiling from source. It also has hard to turn off auto-updates, and spies on you (archive) in other ways. The UI is better than regular Chrome, but still - avoid.
Made by old Opera developers dissatisfied with the direction it was heading in. Probably the most feature-full browser out of the box, however it also unashamedly spies on you (archive) (for example, through the Piwik analytics platform), and has addon auto-updates that can't be turned off. At least you can actually install those addons...anyway, avoid.
Advertising itself as
A BROWSER SECURING YOUR PRIVACY. THAT’S IT., it actually fulfills the claim...except for one small issue (archive). Google SafeBrowsing is turned on by default, which means it will make connections to Google every so often - but you can easily turn that off. My browser of choice for a long time, and I would still recommend it, if not for...
The Holy Grail of Chrome-based browsers, it goes even further than Iridium. For example, some settings which disallow sending information to Google are not even possible to change, so you won't intrude on your own privacy by accident. Even connections to the Chrome Web Store are disabled. It is completely de-googled, exactly as it claims. Ungoogled-chromium also sets its user agent as actual Chrome (which resists fingerprinting), while Iridium unashamedly advertises itself.
As you can see, I've mostly focused on the privacy criteria while rating the browsers. That's because, since we're relying on addons for functionality anyway, "features" are mostly irrelevant. No browser provides even half of what is available through addons. Of course it would be nice to not have to install a bunch of addons to have basic functionality, but that's just situation these days. We have to wait for Otter Browser (or another) to include the usual addon functionality by default. For now, the competition is between IceCat and either Iridium or Ungoogled-chromium. If you are an all-out freetard, the choice is obvious - otherwise, it probably depends on whether you prefer the Firefox or Chrome UIs - though there are also some small differences in addon availability.