Fun find today: we were getting errors from our code only in Firefox dev edition and up, and also only when the devtools were opened. So obviously I had to make a page that detects when the devtools are opened or closed!
(open in FF dev edition or nightly)
Bugzilla ticket is open: https://bugzilla.mozilla.org/show_bug.cgi?id=1545400#c2
macbeth.cc is one server in the network