I'm not sure how recently the mailto: link broke so I don't know which update messed it up or I'd undo it. Firefox still shows GMail as the mail handler. Windows 10 though will not even show Firefox as a choice in the default mail app. When I click a mailto: link, nothing happens (before the "remember" checkbox was ticked it at least brought up the popup asking what I wanted to do). If I then flick File/Email Link... it opens a new tab with email editor and the proper to: and subject:, but it used to do that automatically without the extra step. Web searching for default apps and Firefox pretty much all talk about default browser.