Thank you all.
Lovesmayfamilis I don't have that in my settings. It may be part of the newer versions of Firefox...

I did try Fedora 34 and 35, and Ubuntu, Linux Mint, Mabox, Manjaro, and a few others. There are several things that just don't work (like the screen blanking out every few minutes regardless of your settings, VLC losing audio (yeah, I did try several other video players), or the repo list just getting corrupted, making it impossible to update), and nobody in their respective forums seems to have a clue. That's why I came back to F30, it just works. The downside is I have no security patches.
O_e_l_e_o: right now it seems to be behaving, but I'll try that if it goes haywire again. Either way, I'm planning to install Open Suse Tumbleweed on another drive later today (let's hope it works), so I should be able to get a newer version of firefox (and all security updates!

), so maybe I can fix the whole problem right away...
NeuroticFish: I haven't tried Opera. I might.
I will be trying Open Suse today on another hard drive, and if it works I will have this hdd available to try other stuff. I'm not using it as a cold wallet because it's getting a bit old, and I'm a bit afraid it might quit on me, but a second "spare" OS could be a good idea...
To make things worse, Virtualbox is not working properly (probably because I haven't been able to update it in months), so I can't test different distros on it. Oh, well...
The thing is I have been using Firefox (with this same exact settings), for years, even on this same computer (yeah, it's an oldie), and never had an issue.
ETFBitcoin: right now I'm not using either. In any case, when I was using them, they did work together without issue. In a future install I think I'll go with Ublock though, as it seems to be a lot more aggressive than Adblock plus.
I am trying to get a new distro, but most seem to be having issues I can't fix, and not having internet access at home makes everything a bit more complicated...
Lucius: I've been (not really "playing") with faucets for over a month, and this issue just showed up in the last week or so. I'm still using Cointiply today, and I'm under 9% CPU usage (not the best, but I'll take it).
Stompix: at the time I posted that, either browser was doing it. Today, Firefox seems to be doing ok.
I am using whatsapp, and have been using it for a couple of years now, as it pays the bills. Hopefully I will stop using it altogether soon enough, but we'll see...
In any case, I didn't have any issues with it before, and I'm not having any issues today.
I'm starting to think it may be because I had a lot of tabs open on firefox, showing prices on binance pairs. Those tabs keep running in the background even when you're not looking at them (which may be the reason why I'm seeing ~10% usage now, instead of the usual <5%, as I still have 4 pairs running).