Looks like you followed the method given here, but there's also a comment about setting Spotlight to recognize *.mozmel.

Hmm no I was not aware of that link. Either way it seems very out of date (it's 12 years old) and refers to a separate .mdimporter — you no longer need that since it's built into the Thunderbird app bundle now.

Something else is broken, not sure exactly what. I believe Spotlight requires actual files on disk to perform any indexing (even if those are just placeholder or metadata files). So I would assume that the Maildir format would be a requirement for this to work.

I found a few threads related to that: https://support.mozilla.org/en-US/kb/maildir-thunderbird https://support.mozilla.org/en-US/questions/1234424 http://forums.mozillazine.org/viewtopic.php?f=39&t=3040022

But, it seems to contradict the 60.0 release notes, which specifically say Thunderbird now allows the conversion of folders from mbox to maildir format and vice versa. This is an experimental feature that needs to be enabled by setting the preference mail.store_conversion_enabled. Note that this functionality does not not work if the option 'Allow Windows Search/Spotlight to search messages" is selected.' "

So who knows. Does this work for anyone?

I expect a Mac user will jump in with some insight, but the fact is this feature is seldom used, as it can significantly degrade TB performance, at least that is the experience on Windows.