How can I manually change the default web browser in 11.10?

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http://askubuntu.com – Firefox is not showing in system-settings - system info. So I cannot use the menu to choose it. (I don't want to seem off-topic but I think a little background would be helpful here: The previous Ubuntu version of Firefox was crashing my system all the time so I uninstalled it. I then manually installed the Mozilla version of Firefox 10 which is running well. But if I click on the Check Now button in Firefox's Advanced Preferences then nothing happens, and the dialog that should appear asking me if I'd like to make it the default browser does not show up.) Is there a config file or something (HowTos)