Well, I wanted my Opera to use Thunderbird for mailto links. Even though I have changed the System —> Preferences —> Preferred Applications in Ubuntu Opera was not recognizing Thunderbird as the default mail client. So it goes.
I had some difficulty tracking down the file to point Opera to but here it is. In Opera open Tools —> Preferences and choose the Advanced tab. From the menu on the left select Programs. Under Protocols on the right find mailto and click the “Edit” button.
When this dialog appears you’ll want to click the “Open with other application” radio button. If you are running 8.04 LTS (Hardy Heron) like I am, you should be able to enter /usr/bin/thunderbird to the left of the “Choose” button. Then click “Ok” and “Ok” to get out.
Now your mailto links should spawn a Thunderbird mail window.