There’s a common specification called WebExtension, which is used by all modern browsers. Firefox had their own API (XUL/XPCOM) before that, but they deprecated it in 2017. Safari also used to have its own system for extensions, but it’s been deprecated since 2019. The Manifest API is a subset of WebExtension, which defines an extension.

Fedora, NixOS and Void need a proper wiki like Arch

Most distros could also learn from Arch and create something similar to the AUR. Nix is going in the right direction.

And I guess almost all distros could learn from Artix and Devuan and reconsider if systemd is the right choice.