Oops - make that Thunderbird version 24.4.0 - I mistakenly looked at Firefox the first time. Everything else is accurate though.

Try starting windows in safe mode with networking and then run Thunderbird with NO compatibility stuff. It is not required at all.

Matt -

 Thanks, I guess i can try that but I'm not sure what that proves or how that will help me.    I can't be running windows in safe mode all the time.   If your looking to see if there is some other software that is interfering how would I go about narrowing it down to what it is?   Also, why wouldn't it still interfere when i run it in XP compatiblity mode?

Simple, if it works in Windows safe mode then the problem is software your computer loads when it boots normally... it is a diagnostic process. I would put money on it being anti virus software as XP comparability mode would be putting it as arms length, it not being in the XP virtual machine your running Thunderbird in.

But Windows safe mode will basically prove that.

ok, took me a while to figure out how to get into safe mode in win 8.1(why do they have to make things like this more difficult!). But I'm in now and I'm running Thunderbird in win8.1, not in xp emulation. So far so good but the problem was sporadic/unpredictable as to when it would occur. I'll run it for a few hours and see - if it doesn't happen then its probably not going to.

 But - lets say it doesn't.  I'll go back to my question on how to determine what is causing the issue?   How do I tell if its McCafee or somethign else?

You just said the magic words.... McAfee.

Are you sure you have the latest build of McAfee. They released some real rubbish over the past 12 months. Some of it was simply so bad Thunderbird did not run. It was the cause of a top ten most frequent crash.

Things you have really needed to do to make McAfee work with Thunderbird.

Disable it's spam filter (It puts everything in the trash periodically for some) Disable email attachment checking (is stops and complains about an NSTEMP file intermitently when sending mail. Exclude the Thunderbird profile folder from scanning.

But most importantly get the latest release as they keep updating it to fix the issue and messing something else. But it has been quiet of late. So I had assumed the latest fix actually worked.

yes, I have seen the 'nstemp' error and wondered how to fix that. Usually the second time I sent it worked but I couldn't figure out what was causing it. How do I disable email attachment checking and exclude the profile from scanning? Thanks!

ok I shut off the spam filter (I think), and disabled email attachement checking. I can only find a way to exlude the thunderbird profile from scanning under 'custom scan' - quick scan and full scan don't seem to have way to do that. Program is up to date as well.

ok - after mentioning all the above, I opened Thunderbird, it loaded 3 mails and I tried to delete the first - and it froze....

See if this article helps.

https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#McAfee

If in doubt ask in a McAfee forum.