I had been using Thunderbird on a PC for about two years before needing to move over to a Mac, OS X Lion, to be specific. Thankfully, you can move your Thunderbird profile from a PC to a Mac with hardly any fuss at all.
1) Open up a terminal window. You should be in your user directory by default. If not, get there. The user Library directory is hidden now in Lion, so you have to unhide it with the following command. Once you run this, you can see it in Finder: chflags nohidden ~/Library
2) Exit Thunderbird if you have it running. In Finder go to Users/[user]/Library/Thunderbird/Profiles/[existing default profile]/. Copy the contents of your Windows profile directory into the existing default profile directory on your Mac.
Open Thunderbird and you're done! I had all my emails, my email server settings still worked, and all was well with the world.
As always, make sure you backup anything you're copying over.