Biggest one is that I had to disable Enhanced Tracking Protection to get the preview to work. Only a new relic analytics script was listed in the panel, but reading further I found ETP probably still presents other gotchas for their container setup (see below).
More of an annoyance, but I'm also getting "Service workers are disabled by Firefox on this page[...]" message logged persistently to the console, with a link[1] that goes some way to explaining the ETP issue above. Everything still works. In fact, the log appears to be associated with a firebase service worker, as the subsequent message (every time) is an error due to a firebase long poll getting blocked for CORP header issues.
I'd add that it was working perfectly last week, possibly prior to a browser update. They seem to be somewhat aware of the issue (see [1] and per instructions printed to the non-working preview panel).