I don't like the default source viewer presented by Firefox, so I decide to use a good text editor (in fact, I'm using the best one ;).
First, type about:config in the address bar and hit ENTER.
Next, type editor in the filter bar. It will show you only entries with the word editor.
A double-click on view_source.editor.path will open a pop-up window where you must type the full address to your preferred editor: /usr/bin/gvim.
Then a double-click on view_source.editor.external will change its value to true.
From now on, GVim will be used by Firefox to show you the page source.