I bit the bullet today and installed Windows 7 on my primary computer. All in all, it was a fairly painless process, though because I was switching from 32-bit to 64 (I can finally use that 4th GB of RAM!), I had to do a “custom” install which means I have to reinstall all my programs as well. For most purposes, it’s very much like moving to a new computer.
One of my gripes with Firefox and Thunderbird is that while the installers for both applications are very helpful about importing settings (passwords, network configurations, etc) from competing products, neither one provides a facility for migrating settings from your previous computer.
Fortunately, with a little bit of knowledge, it’s not overly difficult to do this by hand (which is probably why nobody – myself included – has written an appropriate migration tool).
Disclaimer: I don’t use many plugins, so I don’t know if or how well these steps will work with plugin settings. You’ll probably want to wait until after the migration before installing any plugins on the new system.

Copying your Firefox settings

Start off by installing Firefox on the new computer, all we’re copying is the user settings, not the software installation. I recommend you make certain that both the old computer and the new one are running the same version of the browser. (I’m not certain this is completely necessary, but it’s better to be safe.)
Next, one the old computer, you’ll need to use Windows Explorer to visit the directory where Firefox stores its settings.

• On XP: the path is
  C:\Documents and Settings\<username>\Application Data\Mozilla\Firefox
• On Vista or Win 7: the path is
  C:\Users\<username>\AppData\Roaming\Mozilla\Firefox

<username> is, of course, the username you use to log into the computer. (You can find this by going to a command prompt and typing “net config workstation”.)
The “Application Data” or “AppData” directory is usually hidden, so you may have to blind type the path into the Explorer address bar.
On the old computer, make a copy of the “profile.ini” file and the “Profiles” directory.
On the new computer, go the corresponding path on the new computer, delete the existing “profile.ini” file and the “Profiles” directory, and replace them with the copies you made from the old one.

Copying your Thunderbird settings

Start off by installing Thunderbird on the new computer, all we’re copying is the user settings, not the software installation. I recommend you make certain that both the old computer and the new one are running the same version. (I’m not certain this is completely necessary, but it’s better to be safe.)
Next, one the old computer, you’ll need to use Windows Explorer to visit the directory where Thunderbird stores its settings.

• On XP: the path is
  C:\Documents and Settings\<username>\Application Data\Thunderbird
• On Vista or Win 7: the path is
  C:\Users\<username>\AppData\Roaming\Thunderbird

<username> is, of course, the username you use to log into the computer. (You can find this by going to a command prompt and typing “net config workstation”.)
The “Application Data” or “AppData” directory is usually hidden, so you may have to blind type the path into the Explorer address bar.
On the old computer, make a copy of the “profile.ini” file and the “Profiles” directory.
On the new computer, go the corresponding path on the new computer, delete the existing “profile.ini” file and the “Profiles” directory, and replace them with the copies you made from the old one.

Other notes

• I don’t know why Thunderbird doesn’t store its settings under the Mozilla directory. Presumably this is a philosophical difference between the two projects.
• Everything I’d read about a “custom installation” of Windows 7 had me thinking that the hard drive would be reformatted. I was pleasantly surprised to find that this is not the case. (Doing a backup is still highly recommended!)
• Consequently, in the case where the “old computer” is the same physical box, you can find the old “users” directory stored in C:\windows.old. Presumably this would also apply to the “Application Data” directory when upgrading from XP (XP users must do a custom installation when updating to Windows 7). My upgrade was on a Vista machine, so I can’t say for certain what happens with XP.