Mozilla Firefox and Thunderbird as default browser and mail client
You had 2 questions.

1- Go to your Firefox preferences and uncheck the Always check for default browser.



2- For Thunderbird do this:


  Set Thunderbird as the operating system default

Launch the default Mac OS X email application (called "Mail", located in the /Applications folder.)
Select Mail | Preferences from the menu.
On the General tab, select "Thunderbird" from the Default email reader drop-down list.
Close Mail.