firefox 56支持旧式扩展，这很科学~~
它却自动更新，简单地关了也不行，很是牛氓！
==========
-备份C:\Users\用户名\AppData\Roaming\Mozilla\Firefox\Profiles 文件夹。
-卸载FF57，重启，用everything搜索所有ff残留文件，删除，安装56.0.2，断网运行FF，关闭FF 将配置文件覆盖回。
-断网运行FF，设置里关闭自动更新，about：config里false以下3项： app.update.auto app.update.enabled app.update.service.enabled
-还有一个重要任务：
- Edit the file prefs.js, located in %APPDATA%\Mozilla\Firefox\Profiles\xyz.default\ which usually translates to C:\Users\username\AppData\Roaming\Mozilla\Firefox\Profiles\xyz.default\ (Note: username and xyz will vary)
- Make sure the line user_pref(“app.update.enabled”, false); is present in the file; if it isn’t, add it or modify it. Ignore the warning that this file should not be edited
- Save and close the file. OK.
- Optional: Note the button “Check for updates” in the “About Firefox” dialog box. Of course, DO NOT click it until you are prepared and ready to receive the update
由于56 到57 有重要安全更新，所以安全起见，还有一条路，用firefox的一个分支： https://www.waterfoxproject.org/downloads