Source: https://cobaltstrike.net/2022/03/29/chinese-firefox-users-cant-install-ad-blockers/
Users of the Chinese and international versions of the Mozilla Firefox browser in China have reported problems downloading extensions to block ads.
When using the IP address of mainland China to search for four plug-in ad blocking extensions, such as uBlock Origin, AdGuard AdBlocker, AdBlock for Firefox and AdNauseam, an HTTP error 451 appears and the message “This page is not available in your region”.
According to the Landian resource, the situation may be related to some legal problems. Between 2016 and 2018, Chinese companies sued Mozilla. The accusers included Hunan Happy Sunshine Entertainment Media Company, Mango TV, Kuliu.com Beijing Information Technology and Ku6 Media (a subsidiary of Shanda).
The reason for the lawsuit was that Firefox could block pre-advertising of these resources after installing ad-blocking extensions, thereby harming companies.
The lawsuits were filed several years ago, but it is not known why these extensions have now been blocked.