Oct 05, 2017 · Recently firefox changed its policy for addons. So after the upcoming version 57.0 of firefox, Some addons with "legacy" tags, will not work with firefox anymore. And "Proxy Selector
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Proxy Switcher (WebExtension) :: add0n.com
How to Configure a Proxy Server in Firefox
Automatically switch proxies in Firefox's Private Browsing May 21, 2015 2 Ways to Configure Proxy Server on Firefox for Android