Noone seems to have mentioned extensions yet so I'll jump in - extensions!
Like Firefox, Thunderbird can be, uh, extended with additional functionality which can really boost productivity. (I know there are Gmail addons that can do similar things, some of which are pretty powerful too.)
Some ones I use:
- ThreadViz[1] - a neat little visualisation of the email thread & its linked replies/predecessors (pre-plies?)
- Mail Merge[2] - powerful mail merge util with a bunch of options for tailoring emails.
- QuickText[3] - keyboard shortcuts for common (template-able) replies - super useful if you are, say, monitoring a support mailbox and want to save a bunch of time with canned replies.