The "Email This!" extension hasn't been updated for Firefox 57. As a temporary workaround until another one comes along, would you consider: '''Gmail bookmarklet''' A bookmarklet is a script. You add a button to either your Bookmarks Toolbar or Boomarks Menu (as you prefer) to run the script. Then when you want to call up a new Gmail compose window with a link to the currently displayed page, click that button. The script will read the page address and title and open a new tab sending that information to Gmail. ''It won't run on sites that use "Content Security Policy" to block external scripts, but hopefully that will be a very rare problem for you.'' You can install it from here: https://www.jeffersonscher.com/res/sumomarklets.html#GmailLink