Thunderbird uses https://profiler.firefox.com/from-browser from the developer toolbox. This has recently stopped working. There appears to be user-agent string sniffing happening, as when Thunderbird's user-agent string contains Firefox/x.y (which it currently does in nightly builds) the page works.
Thunderbird uses https://profiler.firefox.com/from-browser from the developer toolbox. This has recently stopped working. There appears to be user-agent string sniffing happening, as when Thunderbird's user-agent string contains Firefox/x.y (which it currently does in nightly builds) the page works.
See also: https://bugzilla.mozilla.org/show_bug.cgi?id=1920387