'''Firefox starts blocking uBlock Origin and other ad-blocking add-ons in China on addons.mozilla.org ?''' Recently [1], some users reported that uBlock Origin [2], AdGuard AdBlocker [3], AdBlock For Firefox [4], and AdNauseam [5] were restricted from being accessed by users in mainland China. When users in mainland China try to access the above ad-filtering add-ons on addons.mozilla.org, the website returns 451 Unavailable For Legal Reasons instead of the normal 200 OK response, and the body of the page states "This page is not available in your region". The facts can be validated by reproducing the test results below. By the way, this has nothing to do with China's "great firewall". This action of blocking was performed by Firefox in China. Firefox in China has a long history of arbitrarily blocking websites even for non-political or legal reasons and was found guilty by the Court in China back in 2018 for several such cases. '''Sources and evidences:''' [1] According to Blocky's test logs, restricted access initially began no earlier than March 8, 2022 and no later than March 19, 2022. Users report: https://bgme.me/@bgme/107986773834355287 '''Testing results''' (by blocky.greatfire.org) Before: https://blocky.greatfire.org/api/url_test_result/18800410?format=ooni After: https://blocky.greatfire.org/api/url_test_result/18861443?format=ooni The same testing results can be reproduced by testing the below URLs from a China mainland IP address or blocky.greatfire.org which uses multiple China mainland IP addresses in different cities. '''Affected add-ons: ''' [2] https://addons.mozilla.org/zh-CN/firefox/addon/ublock-origin/ [3] https://addons.mozilla.org/zh-CN/firefox/addon/adguard-adblocker/ [4] https://addons.mozilla.org/zh-CN/firefox/addon/adblock-for-firefox/ [5] https://addons.mozilla.org/zh-CN/firefox/addon/adnauseam/ Court cases: （2017）京0101民初4609号 （2018）京73民终433号 （2017）京0101民初4608号 （2018）京73民终397号