VPN vs. profiles vs. account

I'm a bit puzzled here. Initially, I tried to use the new built-in VPN for only a single domain. That doesn't work at all. Until it does, I tried to get it through other ways. Containers don't work, private windows neither, they both just inherit the global VPN setting. So I moved on to profiles. The web found me a Mozilla support topic that suggested to open about:profiles and create a new one. Did that, opened a window for it, but couldn't get it to reopen on any easy path, just through opening about:profiles and clicking one of the many buttons again. Then I found another description that used the "Profiles" menu. Never seen that before (probably because the menu bar isn't visible by default). Created a new profile there, too, and opened it. After restarting Firefox, all was gone! My original profile with everything was totally inaccessible, like a full data loss. Google search AI in a Chrome browser helped me out and explained what I need to do (in about:profiles, select the old one as default, then restart) to get my environment back. After that I deleted all newly created profiles and then their directories again. Started over with a new "new" profile and continued there. Now I wanted to activate the VPN in there. It asked me to log in again, which was a good sign because finally I got a Firefox window that didn't know about the other window. But then the next failure: I couldn't log in with my credentials because that profile was already used in the other window/profile. Again, they're not completely separated from each other and there's some hidden link between them. So, do I need a new Firefox/Mozilla account only for a separate Firefox profile? I can happily use my one account on different devices but not on the same device? Can I unlink the main profile from my account again? I don't see a logout button anywhere, I'm just logged in invisibly.