Starting with Firefox 57+, only WebExtension Add-Ons will run in the browser.
RequestPolicy needs to be ported to the WebExtension API. You want to help
with this transition? The first thing you can do is to install the
“Development Channel” version  of RequestPolicy Continued and to report any
bug you encounter with this specific channel. Thank you.  RPC dev-
channel: ~on the bottom of page https://addons.mozilla.org/en-
US/firefox/addon/requestpolicy-continued/~ [Edit, 2018-04-04]