Ok, here's what I did to make it work... First of all, do NOT use Get Mail Plus Version 3.0. Use Get Mail Version 2.5.4. Make sure that Thunderbird is set as the default email client both in the program and in KDE.
Next make sure your mailto links work by checking about:config.
(1.) Open Firefox, type "about:config" (no quotes) in the address window, and click enter.
(2.) Right click on the window and choose <New>, then <String> from the pop-up menu that appears.
(3.) In the first pop-up box, enter: "network.protocol-handler.app.mailto" (no quotes, and it might just be easier to cut 'n paste this into the box).
(4.) In the next pop-up box enter the path to Thunderbird, or another email client (e.g. "/usr/bin/mozilla-thunderbird").
That should do it. If not:
(1.) Verify that your path to the email application is correct (e.g. "whereis mozilla-thunderbird" from a console window)
(2.) In about:config verify that "network.protocol-handler.external.mailto" is set as Boolean / True (that's the default AFAIK, so this shouldn't be set to False unless you've changed it).
From what i have deduced, Firefox and Thunderbird are obviously meant to be used together, but an annoying detail is that under Linux they are still not setup that way.
Hope this helps.