I can't remember how long ago it was that some Thunderbird developer thought "wow, tabs! They're great in Firefox, so we absolutely have to implement them in Thunderbird too". Since then I've put up with them in the vain hope that either I'd get used to them, or the developers would change their minds and implement some sort of sane behaviour.
Yesterday I finally lost patience and managed to revert everything manually. This is how I did it:
- To make email open in a new window instead of a tab, go Options > Advanced > Reading & Display and choose "Open messages in a new message window".
- To turn off the overzealous search features, go to Options > Advanced > General, and uncheck "Enable Global Search and Indexer".
- To get rid of the tab bar entirely, go to Options > Advanced > General > Config Editor, promise to be careful, search for the "mail.tabs.autoHide" option, and set it to "true".
- After that, I chose to remove the reply/forward/etc. options from the bottom panel and put them back into the toolbar at the top, via the "Configure" right-click-menu option... that's more personal preference, though. Tweak away!
And now I have a usable mail program again.