I came accross two great Thunderbird add-ons:
Colored Diffs was mentioned on the TYPO3 dev list as a solution for syntax highlighting the diffs attached to posts on the core list. It's realy a great help.
When I was looking at the other Thunerbird add-ons, I also came accross Quicktext. You can use Quicktext to create templates for your posts. This is great if you often post a RFC, bugfix, feature request or a FYI. You just create those templates and pre-fill your new post with one click.