前一段时间在 macOS 上安装了 Avira 反病毒软件，后来嫌拖慢系统，用官方的反安装程序给删掉了。但 Avira 自动给 Firefox 安装的扩展（Avira Browser Safety）却没有被删掉。Avira 官网给出的方法是在 Firefox 删除，但实际操作却发现新版 Firefox（>61.0）竟然不允许在浏览器中删除第三方安装的扩展，清除 Firefox 配制文件也没能把这个插件删掉插件。更诡异的是，在谷歌上竟然搜不到相关的资料。没办法，只好自己手动寻找插件，最后在/Library/Application Support/Mozilla/Extensions中发现了相应的扩展，手动删除后，一切终于清净了。

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Recently I installed Avira AntiVir on macOS, but soon uninstalled on account of slowing down the system. Nonetheless the Firefox extension installed by Avira, Avira Browser Safety, was not been deleted. Avira official website recommended user to uninstall it manually inside Firefox, but the truth is newer Firefox (>61.0) just forbids uninstalling extensions installed by other software from the browser, clearing Firefox data is useless. More strangely, I searched on Google but didn’t find anyone suffered from this issue except for me. Then I have to find the extension manually. It locates at /Library/Application Support/Mozilla/Extensions. The problem is solved by deleting the directory of the extension, and everything works fine now.