I was thinking of posting this, but never got the time. Hope this helps...
First, Welcome to TE. 
Have Fun 'n enjoy.
Backing up the entire Mozilla profile :
(it doesn't matter whether u use FF alone or with thunderbird, just omit whatever is related to thunderbird, if u don't need it)
1. Close all Mozilla programs, including the Mozilla Application Suite, Firefox, and Thunderbird.
Don't forget to exit Mozilla Suite's Quick Launch if it's open (on Windows, it's icon will be in the system tray) and other small window such as Downloads
2. Find your profile (see notes) folder. Go up until you see the folder that contains the folder "Profiles" and the files "
pluginreg.dat", "
registry.dat", and "
profiles.ini" (not all files may exist).
Go up once again and
copy this entire folder. For the Mozilla Suite, Firefox, or Thunderbird, this folder's name should be "Mozilla", "Firefox", or "Thunderbird", respectively.
Copy (but not move) your entire profile folder to another location.
Now, restoring your backup profile:
1.
Copy the
backup "Mozilla", "Firefox", or "Thunderbird" to the
exact same location it once was. The entire directory structure must be identical for it to work.
If you are moving from one OS to another (
as u said, from xp to 2k or viceversa)or
if your Windows hard drive letter (for example, "C:\" and "D:\")
has changed, then u will hve to create a new profile by launching the application after installation, then move the backup profile folder to here and edit the "
prefs.js" file to reflect the new profile folder location and name.
Find "prefs.js" in your new profile folder, open it with a text editor such as Notepad, and replace all instances of the old profile folder location and name with the new profile folder location and name. For example, replace all instances of "C:\\Documents and Settings\\WindowsUserAccountName\\Application Data\\Thunderbird\\Profiles\\********.default" (normal location of the default profile on Windows XP) with "C:\\NewLocation\\NewProfile" (the new profile location). When finished, save the "prefs.js" file.
2.
Specific files of interest
These files are located in your profile folder and may be useful if you are only looking to backup specific pieces of information rather than the entire profile OR can be used to update the profile after a backup/restore of the original profile).
ALL THESE FILES MAYBE BACKED UP AT REGULAR PERIODS SINCE THEY CAN CHANGE WITH TIME FOR OBVIOUS REASONS
1. bookmarks.html  Bookmarks.
(regular backing up is recommended, extensions are available which can automate this task)
2. prefs.js â€â€
Current user preferences.
3. user.js â€â€
User-added preferences (overrides prefs.js every startup).
4. cookies.txt â€â€
Cookies. (for autologin etc)
5. hostperm.1  Cookies permissions, image permissions, and extension website install permissions list.
6. signons.txt  Saved passwords. Requires the "key3.db" file to work.
7. key3.db  Supplements password file.
I strongly suggest to backup 6 & 7 separate and encrypt (for added security) as they contain all passwords saved by the firefox password manager.
NOTES:
1. Where is my profile folder Located?
By default, profiles are created in the folders listed below.
When creating a profile with the Profile Manager, you can choose to place it elsewhere.
For Windows 2000 and XP users;
If you cannot find your profile folder: Windows 2000 and XP sometimes hide the profile folders because "Application Data" is a hidden folder. In Explorer (i.e. double-click My Computer) click: Tools -> Folder Options -> View (tab) -> Show Hidden files and folders.
For those curious, the "%APPDATA%" phrase for Windows 2000/XP is an environment variable that automatically updates for login username changes and will quickly locate your Application Data folder.
Hope this helped you....
