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.
Mozilla also runs hiring and HR for MZLA. They control who gets hired and fired.
It is more like money laundering, than independent entity.
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).