Spectrism

Dark Reader can do this, though it requires a little bit of tinkering. First you need to tick “Enable on restricted pages” in the Advanced section of Dark Readers settings (in the old design the settings can be found under “More > All Settings”). Then in about:config, all entries in extensions.webextensions.restrictedDomains need to be removed and privacy.resistFingerprinting.block_mozAddonManager needs to be set to “true”. If some of this doesn’t work, there’s also a GitHub Discussion with different solutions, but what I wrote here should do the trick.

Why is that ridiculous? Seems like a totally fine solution to me. Probably not possible in most cases due to licencing issues, but if not this is the best thing a developer could do. And making games and/or their servers open source isn’t even the only option. In most cases it will suffice to just provide server binaries and patch the game to make it work with self-hosted servers, or just patch it to make it playable offline. It’s that simple. Developing games with that in mind from the beginning makes this even easier.