In Kubuntu the default browser is Konquerer, but as a longtime Firefox user I wanted to use that as the system default browser. Googling around, I found a way to switch to the browser in Thunderbird, so that links in mails opened in Firefox, but it didn’t change in desktop links and other applications. Now another switcher found the solution. Acutally there isn’t much of a solution in it. It’s more a configuration thing, once you know where to fix it. In the /etc/alternatives/ there is a bunch of links, which points to default applications in several different areas.
To switch the default system browser, change the symlink named x-www-browser from a konqeror link to a firefox link instead and you have a new default web browser across you (k)Ubuntu.
If you feel uneasy changing the links, there even is a small script, which can help you make the changes – update-alternatives.