• Firefox > Web Developer > Page Source
• Tools > Web Developer > Page Source (Ctrl+U)
• Right-click on a web page to open the context menu and select View Page Source

That would be helpful except that it does not open the page in the external page source editor that I have set in config. It still opens it in the default Firefox page source viewer which is lame and useless for editing!

You can first open the source code in a Firefox tab with the view-source: protocol and then use View Page Source (Ctrl+U) to open the source in the external editor.

That does not work, at least on my installation. It merely opens the source page in a second Firefox source viewer window and not in an external editor window. I have the path to my Notepad++ escaped as suggested elsewhere but it does not get called or opened.

(fixed for Southerner as per: [/questions/939775#answer-406824])

As Cor-el said, it is working now.

that worked. No changes that I made in about:config seemed to make much difference but editing prefs.js directly as you suggested (for 64 bit Windows 7):

user_pref("view_source.editor.path", "C:\\\\Program Files (x86)\\\\Notepad++\\\\Notepad++.exe");

The config file shows the path to Notepad++ as being identical to what I had already entered! That entry is and was: C:\\Program Files (x86)\\Notepad++\\Notepad++.exe

It is ironic that entering it in Prefs.js made it work but putting it in via the about:config page did not.  Of course in prefs.js there are two back slashes for every one shown above.

Oh well, at least now it works.

Thank you.