Change Default Text Editor in Fx

By default Firefox uses Notepad 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: CyberNet Forum