To change the default browser, email reader or terminal your system uses, and if you are using Gnome the way to do that is to change the preferred applications.

This was particularly helpful for me when I use Ubuntu as I prefer Thunderbird as my default email reader and not Evolution which is the default in Ubuntu.

well to change this defaults just go to On Debian:

Desktop -> Preferences -> Preferred Applications

On Ubuntu:

System -> Preferences -> Preferred Applications

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Then you will see a dialog box like this, where you will be able to change you preferred applications.