Ubuntu has a simple way to configure the default browser and mail client. Of course, Firefox is the default application for web browsing, so you shouldn’t ever need to use this screen, but if you insist, here’s how to do it.
Navigate to System \ Preferences \ Preferred Applications.
Here you can specify which application you want to use for web or email links. For instance, if you are using Thunderbird for email instead of evolution, you can change that setting here.
You can also choose to automatically open all new Firefox windows in a new tab, although that’s probably the default in Firefox 2.0.
You can also set Gmail as your default email client.