Never heard of that one, I was telling somebody the other day about IceWeasel. So there definitely are FF clones. Or I guess you could just compile Firefox yourself and remove the denylist portion of the code. Would be extra funny if they compiled a version specific for France (because why block sites for everybody else?) and put it next to the regular one on their website with text that said oh BTW if you're in France definitely only download this version, wink wink.

and the source code for both of them is available.

Could just compile yourself without the filtering

But the same applies for the other workarounds mentioned doesn't it?

You should just build one yourself, put it on a disc or flash drive and install it for them.