Change Default Text Editor
Changes which text editor Firefox uses by default

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Tweak Type:	General
Fx 2:	YES
Fx 3:	YES
Fx 3.5:	YES

By default Firefox uses its own viewer when viewing the source of a page (CTRL+U or View, Page Source or selecting View Page Source from the Context Menu). You can select which text editor (such as Notepad++) Firefox uses by default with this simple tweak:

1. In a new tab type about:config in the address bar
2. In the filter field type view_source.editor.external
3. Double-click on the entry to set it to true
4. In the filter field type view_source.editor.path
5. Double-click on the entry and in the pop-up box enter the path for your text editor
6. Click OK and close the tab

Source