This problem had me stumped, as I didn’t know how to call a text editor to view and edit source files, eg to trouble shoot html files. Firefox has View/Page Source in its menus, but you can’t edit the file. After downloading some extensions, which didn’t work because I didn’t know the path to a text editor. Then through some experimentation I found out how it works. Iwanted to use the Cream text editor (I’ve since changed that to Geany, which I find great) but you can use any text editor. I’ll use Cream as an example here.

In the Firefox address bar you type
about:config

in the filter (the address bar) type

view_source

in
view_source.editor.path
right click under ‘value’ and choose modify.
Enter
/usr/bin/cream

That does the trick! It seems that /usr/bin is where the program execs are.