Mozilla fundamentally can't block ublock because Firefox is FOSS. All the other mainstream browsers are proprietary. Firefox is licensed under MPL.
Some extensions get removed from the Firefox extension site, such as addons that promote copyright infringement, because Mozilla can't legally promote that stuff. You can however still install extensions from files.
Even if the main Firefox browser executable from Mozilla were to block ublock extension, then people could legally fork the Firefox code to make their own browser.