Normally upgrading thunderbird changes nothing, it may be a little faster, the odd tweak here or there, not with Thunderbird 3.
Thunderbird 3 is a huge memory sap, it continuously downloads emails if on IMAP, and frankly kills your PC (please note I have not tested on a MAC). Every time I turned on thunderbird it downloaded the same 1000,s of emails, even if it had already done the job before.. The easiest way of avoiding this (they say) is to turn off Global Indexing in the Options (General Tab). Did this work? No. So I decided to revert back to Thunderbird 2.
This is very very easy to do, takes just two minutes.
Firstly, click on Start/Control Panel/ Programs/ Uninstall a program – click on thunderbird 3 (you will not lose any of your files, old emails or settings). Once this has deleted from your pc, go to this page http://releases.mozilla.org/pub/mozilla.org/thunderbird/releases/, this lists past thunderbird versions. The one you want to download is the latest version 2, click on it, select win32 (if you use windows) and then select your language. Now it will download and install.
et voila. fixed.