Lots of web developer do look at source when they do their development.

It is very easy to set the default source editor/viewer in MSIE, but personally I do not like to do my development on MSIE, as in many cases the tables/css does not align/work correctly, debugging scripts are difficult…(chop 30,000 words on the dis-advantages..)

One thing I cannot find in the settings and would like in Firefox is reading the sources with external editor instead of the default source viewer of Firefox….

Found on Firefox help and support page:
By entering about:config in Firefox address bar, user can set hidden/internal settings of Firefox.

Set view_source.editor.external to true. Set view_source.editor.path to your target source editor, example:
“C:PATHYOU_SELECTED_EDITOR.EXE”

And it should do the magic work!

Link
Firefox Tips