I think they’re just kind of implying that RFP is only fully useful when used with Tor Browser (and, it should also be the case with Mullvad Browser and a VPN I’ll point out). You should first understand two definitions:
Advanced scripts are scripts that actively try to detect people using anti-fingerprinting techniques and fingerprint them anyways.
Naive scripts are scripts that fingerprint you based on reading some values from your browser.
Naive scripts account for pretty much all tracking scripts you’ll encounter on the web today, unless you’re like being hunted down or something.
Now there are 4 facts:
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RFP with Tor browser can thwart even advanced scripts.
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RFP with Firefox can’t really thwart advanced scripts, because there are still plenty of other factors that they can use to fingerprint you.
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Both RFP and FPP provide good protection against naive scripts.
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FPP is more compatible with websites than RFP.
Given all of this, they have made the choice to default to FPP, because it is more compatible, it is more flexible, and in Firefox/Arkenfox you can only realistically thwart naive scripts anyways.
If you stick with RFP you won’t be worse off, but the advantages are so small in this case with Arkenfox/Firefox that you’re also not likely to be better off either, so the trade-off of using RFP instead of FPP is probably not worth it.
Anyways, this is why we say that for people with the highest levels of risk, they need to be using Tor Browser. For people who are still concerned about these advanced threats, but can accept some tradeoff for performance (because the consequences for them might not be high, perhaps) they can use Mullvad Browser with a VPN. And for people who are only concerned about naive scripts (which is most people!) they can use Arkenfox/Firefox.