The reality however is that for whatever reason most people don't have good backups, to make things even more serious many assume that the windows system restore will do it for them. Windows system restore is almost as likely to leave you will a non functional Thunderbird as it is to do anything.
Leaving aside my personal anger at the decision Microsoft made 10 years ago that home users just don't need backups, and subsequently excluded the backup program from the operating system you purchased. You find yourself in the situation that your hard drive crashed, the nice person at the computer store has put your old hard drive into an external case you install Thunderbird and all your mail is simply not there. To recover from this situation, you need to role your sleaves up and get into the nity gritty of the file system on your old had disk.
I am going to talk here primarily of windows, as Linus and mac actually have build in backup programs and in the case of iCLoud, a nice secure place to store it.
Under Windows Thunderbird profile data is stored in a collection of folders in a Hidden windows folder called Application Data. You will find all sorts of references on the web telling you to use the %appdata% windows environment variable to locate this folder, but that will only work on the current operating system, not your old hard disk. So to start with there are some changes you need to make to windows.
First you have to tell windows to show hidden files and folders.
I have linked to Microsoft here, as it is their operating system. If their explanation of how is not enough for you, please ask Microsoft.
Now we need to locate the Application Data folder, but first a convention [username] means your login name to the computer.
Windows XP the folder is usually C:\documents and settings/[username]\application data
Windows vista it is usually C:\users\[username]/appdata or C:\users\[username]\appdata\roaming
Windows 7 is is usually C;\users\[username]\appdata\roaming
Hopefully this image I stole from the Microsoft site will explain a little
Once you locate the application data folder, it is simple to then locate the Thunderbird folder it contains.
The Thunderbird folder contains a file called profiles.ini. This file actually containsa couple of thing that we need to note. One is the location of the actual profile folder. So open the file in wordpad. Wordpad will format it nicely, notepad will make a a long string of gobbldy gook
[General] StartWithLastProfile=1 [Profile0] Name=default IsRelative=1 Path=Profiles/it39phn2.default Default=1 [Profile1] Name=tester IsRelative=1 Path=Profiles/zwldg1rp.tester
The two things of interest are the IsRelative and Path.
IsRelative =1 means the profile is located further down the Thunderbird folder tree. if it is not =1 then the folder will be located absolutely at the location pointed to in Path=
Unless you have changed defaults, IsRelative will equal 1 and you need to navigate down the folders until you locate to place pointed to in the path= line.
You have now located your old profile folder. The question arises, do you just want some of the information out of it, or do you want it all as a replacement for what you have. The latter option is by far the easiest.
Simply replace the files in your new profile folder with the ones in your old profile folder and start Thunderbird and all will be back as it was before.
If replacement is not an option, I would suggest you install the Import / Export tools to Thunderbird and export the new mail you have in your new profile as EML files. Replace the files as above and use the Import Export tools to re-import the new mail..