@Toad-Hall I have my own Thunderbird set up exactly as you described and if I double click an email it opens in a tab. Changing the sort to threaded or unthreaded makes no difference. If you double click an email it opens that email in a tab.
I have several older client that use Thunderbird. I regularly get calls from them saying their email is not working or they can't get emails. What happens is they open messages by double clicking them and get so many tabs opened that they get "lost"...
I am really disappointed that Firefox has fallen behind the other browsers on this. Firefox is supposed to be the more secure and better browser. Passkey compatibility should have been a no brainer.
