I use Firefox & Thunderbird as my main browser and mail client (respectively!) They’re both useful enough out of the box, but loads of people have published even more useful extensions which “enhance the user experience” as the blurb goes. I have a number of other addons installed, but my essentials are:

Thunderbird

• Nostalgy - forget about the folder window; just a few keystrokes will do everything you need.
• Correct Identity - I serve a number of email addresses (from completely different domains) through one mailbox. This extension makes a good guess as to which hat I should be wearing to reply to an email, based on the “To” address.

Firefox

• Sage - no matter what other RSS reader I try, I keep coming back to Sage’s simplicity. It lacks a couple of things I might like, such as the ability to vary the refresh interval by feed, but it does what it does simply and well.
• IE Tab - indispensable if you’re developing web pages or just trying to view the ones out there.
• CSS Viewer - not as powerful as Firebug, but very handy for a quick check as to what’s going on with your page’s styling.