Gmail's recently announced IMAP support brought us closer to Thunderbird/Gmail sychronicity nirvana - but as Lifehacker reader Dan pointed out, you need to do some tweaking to get access to your Gmail contacts in Thunderbird.
Install and set up GCALDaemon (just remember to 'install' and 'start' the service - that had me stumped for a while!). This gives you the ability to search for contacts when creating new emails.
Enter '*' (without quotes) into the contacts search box and you'll get a full list.
Add the Contacts Sidebar Add-On though and its default setup will automatically download your Gmail Contacts for you to view whenever you want (as long as you're online).
Thanks for the tip, Dan!