When you select "Web Developer --> Page source" in Firefox you get a view of the HTML source for the web page. This is not an editor (by default).
If you carefully follow these instructions you can get Firefox to pop up that HTML source in an actual editor instead of a view window. Then you can directly edit the HTML source and save it back:
1, type "about:config" in the URL window at the top of Firefox
2. It will give you a cute message about voiding your warranty... this
is simply to make you believe you should be careful about changing
the default configuration settings in Firefox... you don't want
to really bollix things up so be careful
3. search for "source". This should cut the list to a dozen-ish lines
4. Look for the line that says
view_source.editor.external
It will be currently set false. If you click on it, it will
highlight and change to true.
5. Now find the line that says
view_source.editor.path
It is probably just under the line you changed to true.
Click on the last field and it will pop up a text box where you
type in the path to the editor you want to use for editing HTML
source.
On my machine, I gave it this
C:\WINDOWS\system32\notepad.exe
This makes Firefox use notepad to display the HTML source and then
I can edit it and save it. Then I can refresh my browser and re-run
the JavaScript program I am working on.