Is Global Search enabled in Settings/General/Indexing? If you have large folders, the db rebuild can take a while (monitor progress in Tools/Activity Monitor).

See also https://support.mozilla.org/en-US/questions/1176987

Is Global Search enabled in Settings/General/Indexing? If you have large folders, the db rebuild can take a while (monitor progress in Tools/Activity Monitor). See also https://support.mozilla.org/en-US/questions/1176987

Thank you thank you thank you! The "Enable Global Search and Indexer" was the right thing. Initially I didn't see any difference but after removing the global database again, I saw in the activity manager that indexing was ongoing and the database file size was growing significantly. It is still processing but I can already open messages in conversation.

Your quick and on point reply is much appreciated!

@sfhowes, turns out the problem remains. Sometimes I see the open in conversation option and sometimes I don't. It seems like it's either working for all messages or doesn't, probably not a message by message thing.

What I can do to debug this further?

EDIT: For example, right before I sent this message I didn't have the option at all. Moments later after sending this post, the option came back on. I opened the activity manager to see it finished indexing moments before. The account I was checking on was the one with 37 messages indexed on the screenshot.

I can't explain why the the conversation option isn't always working. I don't see the issue here, even with gmail accounts. Try running in safe/troubleshoot mode (hold Shift when launching TB). Also, consider unsubscribing All Mail (see All Mail section), as it is a duplicate of every message. It probably also slows down the search indexing.