Add-ons

• AddressContext -- Allows you to add multiple addresses from an email into your address book, right-click on an email to open their address book page, and other handy little things.

• Check and Send -- Allowed me to set a filter so if the word "attach" is in my email, when I send it reminds me to actually attach the file. It also prompts me to confirm sending to addresses that aren't in my book, so I can add them.
  
• Enigmail -- Adds PGP encryption and signatures to email. See below.
• FolderFlags -- Allowed me to set another folder with the 'Junk' flag, so it's not scanned by Thunderbird for spam.
  
• Lightning -- Integrated calendar, allowing me to subscribe to iCal calendars, and send meeting invites. 
• Provider for Google Calendar -- Allows bidirectional access.
• Remember Mismatched Domains -- I pick up my mail at mail.combsnet.com instead of mail.webfaction.com. This stops Thunderbird from warning me to death.
  
• Select Addressbook Text -- Allows text portions of an address book card to be selected, copied and pasted.
• Signature Switch -- Allows me to have a different signature lines for different mailing lists, or for personal use.
• Sync Kolab -- Synchronize address book, calendar and tasks using an IMAP folder.
• Tag Toolbar -- Add toolbar for toggling tags.
  
• ThunderBirthDay -- Display birthdays from the Thunderbird addressbooks as events in Lightning.
• Zombie Keys -- Makes it easier to type accented letters. Type control-~, then n, to get ñ. (Zombie keys is also a very useful Firefox add-on.)
  

Thunderbird Settings

When you forward emails, are you getting gigantic headers? Set View > Headers > Normal.

Enigmail

You need either the Windows or OS X version of GNUpg.

There are a couple of configuration tweaks you'll want to try:

Better text encryption: If you're finding '>' characters replaced with '|', look at the text=flowed configuration.

Encrypting HTML with PGP/MIME: http://www.e-ignite.co.uk/html/openpgp.html#PGPMIME

Lightning

To subscribe to an iCal, copy the URL to the clipboard. In Lightning, use File > New > Calendar, and then click the "On the network" radio button and paste in the URL.

To set it so incoming meeting invites (sent from someone using Lightning) are clickable, make sure View > Display attachments inline is selected.