Hi MossyRock,
What kind of horror stories? The scariest one I've ever heard is The Tale of the Quarantined Inbox. This can happen with many email clients due to the fact that the Inbox holds all the stored messages as one single file, so one infected message can cause the entire Inbox to be put into quarantine. This can be prevented by adding the Inbox to the Norton Scan exclusions. This does not lessen your security because Auto-Protect will detect any malicious file as soon as an attachment is actually opened (which takes it out of the Inbox). This issue, by the way, is not unique to Norton or to Thunderbird.
Like delphinium, I have used Thunderbird 2 and now Thunderbird 3 for some time with no issues at all. However, I have left Norton Email Scanning enabled on my system. There is an option in the Thunderbird settings to "Allow antivirus clients to quarantine individual incoming messages." This setting makes Thunderbird store incoming messages as individual temp files before actually adding them to the Inbox, thus giving your antivirus program a chance to scan and remove a single infected new message without having to quarantine the entire Inbox. However using Incoming Email Scanning is purely a matter of choice, since, as delphinium points out, from a security standpoint, it is not essential and there are plenty of legitimate reasons why you might want to disable this feature, no matter what AV program you happened to be using.
I also use Thunderbird's Antispam, rather than Norton's.
You should not have any problems using Norton along with Thunderbird 2.
