The hardest part in moving your profiles from Windows to Mac (or Mac to Windows) is finding them in the first place. On Windows, I found my Firefox in C:\Documents and Settings\[username]\Application Data\Mozilla\Firefox\Profiles
where [username] would be your username.
I located my Thunderbird profile in C:\Documents and Settings\[username]\Local Settings\Application Data\Thunderbird\Profiles.
Since I have a network, and Windows shares its system drives by default, the easiest way for me to connect the two systems to transfer the files was from my Mac. In Finder, I pressed Command-k and entered the path to my Windows machine’s hard drive, like so:
where [computername] is the name of my PC. I was prompted for my Windows username and password, and then my Windows computer’s hard drive showed up on the desktop. From there it was a matter of just drilling down into the folders listed above to the Profiles directory.
Then, on the Mac, I had to locate the appropriate preferences folders. I opened my hard drive, then clicked on my username from the icon bar at the left. From there, I opened Library, Application Support, FireFox, Profiles. There I saw a profile named, in my case, ijawgk1.default. On Windows, the folder was named 9u4pqopm.default. The names are not important. What is important, is that you copy the contents of this folder on Windows into the folder on the Mac. Do not copy the whole folder – open the folder under your Profiles folder, select all files and folders underneath, and then copy all of them to the appropriate folder on the Mac. Restart Firefox or Thunderbird, and voila! You should see all of your customized settings, bookmarks, addresses, extensions, etc. on your Mac.
If you want to move your profile from Mac to Windows, simply reverse the copy, copying the contents of the profile from the Mac to Windows.
If you’re using Linux, you can do the same thing. You just need to know that your profiles are located in ~/.mozilla/firefox and ~/.mozilla-thunderbird.