Thunderbird lives in an entity called MZLA, created by Mozilla and the Thunderbird Council (the elected open source community leadership) to be the legal and financial home to Thunderbird.
MZLA, which I lead, receives only money via donations from donors. We are a for-profit because the IRS is very skeptical of nonprofits who develop FOSS software (someone down below linked to a bunch of great links on this topic). We, do, however publish tons of information on what we are doing on our blog, and we are open source and a community run project, so if you really want to go deep on this - come join us in making Thunderbird.
Great links: