Make Mozilla Firefox The Default Web Browser

Open "KDE Control Center" AKA "Configure Desktop / Personal Settings" or hit ALT+F2 and type:  kcontrol

Click the "KDE Components" button on the left panel.

Click the "Component Chooser" button on the left panel.

Click the "Web Browser" item from the selection list area.

In the "Defailt Component" section, check the "in the following browser" option.

In the text field directly below that option, type:  firefox

Click the "Apply" button at the bottom right and close KDE Control Center.

Firefox is now your default browser.

To test it, click a URL in your email or from an IM or on IRC.