> There's one screen that loads up 40,000 rows (crazy you say?) worth of data that performs fine on Chrome and is laggy on Firefox

Firefox's performance engineers are eager for bug reports! You can record a performance profile [1] and file a performance bug report in Bugzilla [2]. It helps if the slow page is accessible to Firefox engineers for testing, but your performance profile is a big head start.

[1] https://profiler.firefox.com/

[2] https://bugzilla.mozilla.org/enter_bug.cgi?product=Core&comp...