Back when I tried it, I only had it in one device--which is great, since I dunno if I can do it on more than one device, let alone worry how a hardened Firefox mobile would even look like.

I actually don't remember if the settings change with updates. But I suppose they don't (as they don't either with Librewolf). What I meant with "hard to maintain" is basically keeping note that the hardened Firefox config doesn't behave like vanilla Firefox (and isn't expected to). Making some temporary changes to accommodate a "necessary evil" website, you'd have to make note what setting you "temporarily" have to change it to, what the hardened config should be for that setting, and most importantly: remembering to change it back to the hardened config.

So, I guess it's not really a matter of maintaining the config than being aware of all those config changes (from default). With LibreWolf, I'm just brushing it off as "yeah, that's how LibreWolf works."