You can view the source code of websites in Firebox by pressing the Ctrl + U keyboard shortcut. That displays the HTML code for the website in a small built-in source viewer. However, you can also view the source code in external text editor software like Notepad.
Enter about:config in the Firefox address bar to open the page in the shot below. Then locate the view_source.editor.external setting on that page. To find it, enter view_source.editor in the Search bar at the top of the page.
There you should double left-click on it to change its value to true. Then find the view_source.editor.path setting. Double left-click on view_source.editor.path to open the Enter string value window in the shot below.
Enter C:\Windows\Notepad.exe. in that window. Click OK to close the window, restart the Firefox browser and open a website page. Now press the Ctrl + U shortcut key to open the source code in Notepad as below.
So now you can view source code in Notepad and alternative text editors. Input alternative folder locations in the Edit string window for other text editors. You can also open source code in external text editors with the Dafizilla ViewSourceWith Firefox add-on.