The thing is, Firefox has an extension API. It's a proper thing that they maintain, and make guarantees about. GNOME intentionally doesn't have an API, because if they did, the things extensions could do would be limited to what those APIs expose. Instead, they let extensions do whatever they want, patching the code of the shell directly. This comes at the cost of extensions needing to be updated for new shell versions, but it lets extensions be extremely powerful.

In fact, Firefox had this issue a few years back. They switched from a GNOME like system to the WebExtensions API, which is more limited and broke basically every extension if they didn't update. There are still some add-ons that can't be replicated because they need functionality the API doesn't expose.