Rated 4 out of 5 stars
It seems not to work anymore with TB 60. Promotion or collapse of folders has no effect, at least IMHO. I hope it could be updated even if I suspect it will be not.
Rated 5 out of 5 stars
I discovery only today this litte jewel.
It is very useful for those who do not like the hierarchical structure folders in GMail under Thunderbird.
I presently use the latest TB version (45.1.0) and FlatFolderTree works perfectly!
You simply need to expand the .xpi file (which is exacly as a .zip file), open its .rdf file (which is a .txt file) with an editor (like notepad or notepad++) and change the label:
to, for example:
Being Italian I also added the translation for my language :-)