https://decodo.com/apps/firefox-proxy-add-on
Firefox Proxy Extension – Try Proxy Add-on
With Firefox proxy add-on, we're introducing the virtual world at your fingertips. Easy to set up, even easier to use. Focus on your tasks without cloaking.
firefox proxy extensiontryadd
https://iproyal.com/firefox-proxy-manager/
Firefox Proxy Manager Add-on (Extension)
proxy manageradd onfirefoxextension