Concerning your thunderbird question: all your mails and mailing settings are stored in your homefolder in
/home/$YOURUSERNAME/.thunderbird). Just backup this folder and paste it in the same direction on a clean install and you are able to continue with Thunderbird, where you left.
The dot in the folder-name indicates that the folder is hidden. You therefore normally won't see this folder. To display hidden folders, you may press
For your documents you would logically backup your Documents folder in your home-directory (however it is on your system called as it is localized).
And to installation faults: I don't know if there is something like a snapshot possibility but if you have installed a wrong package or such and don't see a thing, you may want to remember that you are using a Linux system. Press
[F1] to enter your first tty. This will work no matter if your xserver is acting faulty or not. From there you can revert any changes you made like you would do it in the terminal.