Spotlight does not work with Thunderbird? mdimporter installed but no work

Thunderbird stores mailboxes in single database files and you won't be able to search inside them using anything except Thunderbird itself.

ok this is some extra piece of software which seems to create a separate index for Thunderbird emails outside of the database files which can then be searched by spotlight. I don't use it so can't comment on this directly. one obvious thing is that you'd have to include system files in your search before you see any results from the thunderbird folder because all this stuff lives inside your user Library. to include system files in a search start finder search using command+f, click on "kind" scroll and click on "other". in the resulting popup select 'system files' and change the settings to include them in the search.