Might sound stupid given that you seem much more knowledgeable than me regarding this matter, but have you considered changing resolvers or is that out of the question? Quad9 is a very reputable company (with which I'm not affiliated in any way), and as I said before, never had an issue with them.

Have you tried raising an issue in the mozilla forums or something like that? I enabled it when it came out and I don't remember any problem that occurred because of it. Hope you can fix it!

Yes, I do, but I don't think it was because of this. It's still enabled and now I don't have any problems with the site. Do you think that it's related to DoH?

I knew about the site, but I could not access it either. I've been AFK a couple of days, and without tinkering with anything, it's all back to normal. Thank you for your help!

Thanks, everything is working well, now. I have no idea what happened but it's over.

Sorry for the late reply! Today I tried to enter the site and everything works flawlessly without any change on my end, so everything is ok now! Thanks for your help, dude!

Just occurred to me that I could try the clearnet domain on Tor, and it works well. I don't get what's up with my Firefox installation, it doesn't work in my profile nor in any new profiles.

Coud you post the .onion link, please? I have no way to enter the site to look for it.

Yeah, that's it. When I enter the old .to domain I get redirected to this one automatically, and it gives me the certificate error.