Copy old configuration, including address books. Edit
/home/username/.mozilla-thunderbird (where . indicates a hidden directory: change view if necessary)
Making Thunderbird the default mail client Edit
Linux - Gnome
* Go to "Applications -> Desktop Preferences -> Advanced -> Preferred Applications". * Select "Custom Mail Reader", and type "thunderbird %s" in the box. You should replace "thunderbird" by the full path to the thunderbird executable if it is not in the PATH.
If the above menus are not available to you, another way to do it is through the GNOME Configuration Editor:
1. Run the GNOME Configuration Editor using the command "gconf-editor" (this can be done from a console). 2. Navigate to "/desktop/gnome/applications/url-handlers/mailto" (on some systems, this may be "/desktop/gnome/url-handlers/mailto") 3. Edit the "command" field and change it's value to "thunderbird %s" (or "mozilla-thunderbird %s", depending on your system). If thunderbird is not in your PATH, then specify the full path to the thunderbird executable (I'd suggest just having thunderbird in your path). 4. Make sure that the "enabled" field is checked. 5. Make sure that the "needs_terminal" field is unchecked. 6. Close it. 7. You're done. :-D
I used this approach on SuSE Linux 10.0 under GNOME.  Linux - KDE
Opening a mail client, be that Thunderbird or some other email client, can take place either from within integrated KDE applications and "KDE-aware" applictions, or applications like Firefox that do not check KDE's preference settings. In the latter case like Firefox, one must set those preferences directly in the application.
For both cases, the first step is to determine where in your Linux filesystem the desired mail client resides. You will probably want to write this down since in some cases a Modal window or dialog box will be open, sometimes preventing actions to find the Full Path you need.
In my case, from a terminal window, I ran "type kmail" (without the quotes), though you might wish to enter "type thunderbird". Answer was: /usr/bin/kmail Note: "whereis kmail" (or whereis thunderbird) will return more answers than just the location of the executable "program".
 Setting email client for Firefox in Linux
These steps will allow opening your desired email client in Linux when operating in Firefox and using its menus, or when activating an email link on a Firefox page:
1. In Firefox's Location (URL) Bar, enter "about:config" and then press <enter> or click "Go". 2. With the cursor in the body of the resulting page, <right-click> the mouse. 3. From the pop-up menu, select "New". 4. From the next pop-up menu, select "String". 5. In the pop-up dialog box "Enter preference name", enter network.protocol-handler.app.mailto (without quotes), and click "OK" (You might wish to cut-and-paste that phrase to ensure correct spelling). 6. In the pop-up dialog box "? network.protocol-handler.app.mailto", enter /usr/bin/kmail [or the actual filesystem location of your desired email client] and click "OK". This should be the Full Path of the program, i.e. /usr/bin/kmail, not just the path /usr/bin.
Without restarting Firefox, you can test by opening or switching to another tab. From the Firefox top menu select, "File -> Send Link". Your desired email client should open.
If that doesn't happen, redo your steps, ensuring the spelling of your entries is correct, and ensure the actual Full Path of your desired mail client is entered.
 Setting email client for KDE applications
These steps will allow opening your desired email client when operating in an integrated KDE or KDE-aware applicaion.
We need to open the KDE Control Center. Depeneding upon how your menu system is layed out this might be done through one of several selection paths, starting from the bottom Task Bar:
* "K (start) -> System -> Configuration Editor" * "K (start) -> Configuration Editor"
With the KDE Configuration Editor open, now:
1. Select "Components -> KDE Components -> Component Chooser". 2. In the left list box, select "Mail Client". 3. Check the radio button "Use a different email client". 4. In the text box below that radio button, type the Full Path (Like /usr/bin/kmail), not just the path, to the executable program of your desired email client in the box - be that Thuderbird or something else. 5. At the bottom of the dialog box, click "Apply". 6. Close the KDE Configuration Editor.