Don't recall when the change actually took place, but now on the Gmail page the following pop-up always appears "use firefox to open mail.google.com every time you click a link that opens your email?" The pop-up that appears is the same as in https://www.reddit.com/r/firefox/comments/1g09q1h/how_to_disable_popup_on_gmail_use_firefox_to_open/ Do note that I'm also using thunderbird and it is installed as the default mail handler in windows. I've also changed network.protocol-handler.external.mailto to false in the firefox settings. It has not solved the issue, the question re-appears at every firefox startup.