They are not entirely separate from Mozilla. The MZLA Technologies Corporation is a for-profit subsidiary of the Mozilla Foundation. They have access to some of Mozilla's common infrastructure, but are otherwise entirely funded by donations. Donations to MZLA only fund Thunderbird and no other products.
Seems fine if you can donate to Thunderbird development. Compared to Firefox, where I don't think it's possible to donate to development at all (only to Mozilla activism side).
And both are owned and controlled by Mozilla Foundation, which is the issue. Why on earth would I donate money to an organization that seems dedicated to doing as little as possible other than acting as a tool to be used for the personal benefit of its leaders?