Since I switched to using Firefox and Thunderbird for surfing/mail respectively, I found that they worked differently than IE and Outlook when I came to backing up the data. Some research revealed that I could just copy the whole folder under the username to make a backup copy.
Here is the location where it is on the harddrive:
C:Documents & SettingsusernameApplication DataMozillaFirefox
C:Documents & SettingsusernameApplication DataThunderbird (for some reason this one is not in my Mozilla folder, but could be in yours)
So copy them on a regular basis for a backup copy.
originally posted on 23-6-2007 in the old site.