I don't use Thunderbird so this is only a guess. Assuming this is the latest version of Thunderbird.
Make sure you aren't storing emails in the Inbox as VirusScan scans the Inbox constantly.
Make sub-folders for storage of mail
Click Web and Email Protection
Click Internet Connections for Programs
Click the word Program at the top to arrange alphabetically if you wish
Look dow that list for Thunderbird
Make sure it has full access using the Edit button if necessary.
The firewall effects how the rest of SecurityCenter acts so that might help.
If that doesn't help then I suggest contacting technical Support, it's free by phone or online chat and available through McAfee Support or Useful Links at the top of this page.
I have the same problem. This has taken me two days to discover, but Thunderbird (vers 17.0) is running painfully slowly. Having tried all the suggested work rounds and tips, the solution appears to be turning off McAfee 'Real time scanning'. As soon as it enables, every task in Thunderbird slows to a crawl - 30 secs to delete one message etc. Making an exception of Thunderbird in the Firewall has no effect.
I'm about to contact help online.
I also have this problem after an update this week. The only solution I've found is to turn real time scanning off before checking e-mail which is not really a solution. I deleted a lot of old emails and it's a little faster but still a minute or so for each operation.
I've considered dropping McAfee on a number of occassions - this perhaps is another time. Hoping for a solution soon.
I also sent a request on Friday.
However, I've discovered that with the most recent update I can get Thunderbird working properly by simply unchecking 'email attachments' in the 'Real time scanning' 'Settings'.
That's one way of getting around it and in fact is OK because should any of those attachments be infected and subsequently saved to the hard drive, they would then hopefully be detected by real-time scanning anyway.
The other thing to try is to go into Firewall > Internet Connections for Programs and using the Edit button turn Netguard off for Thunderbird and Save that setting. Not an ideal situation but for the moment it may help.
Hint: Clicking the word Program at the top of that list will arrange them alphabetically.