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.

Because it’s proprietary garbage. If there are FOSS alternatives, I’m most definitely going to use them instead of proprietary software, let alone proprietary software by companies like Meta. And since there are plenty of those alternatives: No WhatsApp for me.