This title is inaccurate. Mozilla is _not_ hiring a developer to work on Thunderbird full-time. See below:
From the post:
"
The Thunderbird Project is hiring for a software engineer!
...
Please note that while the Thunderbird project is a group of individuals separate from the Mozilla Foundation that works to further the Thunderbird email client, the Mozilla Foundation is the Project’s fiscal home. The Thunderbird Council, separate from Mozilla, manages the Project and will direct the software engineer’s work."
From the post:
" The Thunderbird Project is hiring for a software engineer!
...
Please note that while the Thunderbird project is a group of individuals separate from the Mozilla Foundation that works to further the Thunderbird email client, the Mozilla Foundation is the Project’s fiscal home. The Thunderbird Council, separate from Mozilla, manages the Project and will direct the software engineer’s work."