This is a Thunderbird add-on for manipulating internal folder type flags. Mostly it helps mark folders containing server-filtered spam as the Junk type so those messages aren't run through Thunderbird's junk filtering. Flags affect both appearance and behavior.
You should acquire it from addons.mozilla.org. Thanks to the hard work and commitment of folks at BabelZilla, the add-on is available in a wide variety of localizations.
This is a Make based project. There are two main targets, one builds the
XPI appropriate for deployment (all), the other builds an XPI for
translation for upload to BabelZilla (babelzilla), which includes strings
that are used in the install.rdf and addons.mozilla.org and consequently
sit outside the normal flow of translation.
FolderFlags overlays chrome://messenger/content/folderProps.xul and so
adds a new tab, which reads and writes Thunderbird's settings for the
selected folder.