It is firefox with all of the non-free components and telemetry removed.
Why I think this tool should be added
It is a FOSS browser funded by the biggest FOSS organization out there - FSF. Firefox’s TOS changes aren’t as horrible as some make it out to be, yet I think supporting an alternative project secures the future of the Gecko Engine, especially one as dedicated to FOSS and openness as FSF. It also has security and privacy upgrades, like disabling JS and preventing cross site tracking.
They don’t distribute official binaries which means you’re trusting a random third party when downloading this browser, or you have to compile it yourself which I don’t think is reasonable to ask of most people, it’s much better if there’s an official first party binary to download.
Second, it’s based on Firefox ESR. According to Mozilla themselves:
Over time the ESR will be less secure than the regular release of Firefox, as new functionality will not be added at the same pace as Firefox, and only high-risk/impact security patches will be backported. It is important that organizations deploying this software understand and accept this.
I don’t really see how recommending icecat would secure the future of Gecko, Mozilla is the one developing it. The FSF isn’t going to pick up Gecko development if Mozilla goes under.
So given all that I don’t think I’d be comfortable recommending this.
FWIW both the Tor Browser and Mullvad Browser are based on Firefox ESR and both are currently recommended by Privacy Guides. Though I agree the other two points you’ve made should disqualify GNU IceCat.