Firefox has an option to see the generated source code of an webpage by selecting the text in the webpage and then right click on the selected text and select “View Selection Source” from the context menu, but this opens another window with the HTML code the webpage, which cannot be directly edited if needed, but this can be changed easily using the Firefox config.

Here are the steps :

1. In the Firefox address bar type about:config 
2. Search for the entry view_source.editor.external by copy pasting it from here in the filter text box and modify this to true by double clicking it.
    
3. Next, search for the entry view_source.editor.path by copy pasting it in the filter text box and change this to the explicit path to your editor of choice.

The next time you view source of a webpage the source code would open up in the external editor of your choice.