(Up to: Building Blocks )

Firefox

• SessionSaver: A must have for when you need to close your browser down
• without losing all your tabs... (no longer needed with Firefox 2 - yay!)
• Add N Edit Cookies: Easy editing/deletion/addition of cookies. Useful for debugging things.
• delicious: Very nice del.icio.us integration, with support for multiple accounts and everything.
• SwitchProxy: Flick between proxies quickly and easily - useful when Tor is running.

Less "essential", more "interesting"...

• Greasemonkey: Client-side scripts overlaid on webpages. Doing lots of "interesting" things, may become "essential" depending on those scripts.
• Platypus: Example of an "interesting" GM script - highlights HTML elements on mouseover, and lets you alter the style and save the style for next time you visit.
• Annozilla: Swap comments with other web users.

Thunderbird

• Enigmail: PGP or GPG integration with the mail client
• Tagzilla: Fantastic signature-management plug-in
• Virtual Identity: Quickly change your sender details without fiddling in the Profile Preferences
• Bork Bork Bork!: Heheheh. A Classic. (Also for Firefox)

Others to check out in future...

• Message Notes vs Notary: attaching local notes to each message
• MessageID-Finder
• SMSalias: Sending SMS through SMSalias.com (Also for Firefox)
• QuoteCollapse: reduce those mailing list quotes...

(See also: Mozilla Bookmark Merger )