What a shame, when you are used to Notepad or your UltraEdit(or), to have to view the source of your web pages with the viewer included in Firefox. However, it is easy to change this, and you don’t even have to restart your browser for changes to take effect. Even when you use the default Firefox hotkey (Ctrl+U or Ctrl+Shift+U) you will be able to view the source with your chosen editor.

To the address bar of Firefox write, about:config

A “Filter” field will appear; write there (or copy from here and paste): view_source.editor.external

Set the value to true (double click), and in the “Filter” field again, write, view_source.editor.path

Double-click on this line and enter the path to the source viewer you’d like to use.

Enjoy!