I need to customize some error pages and this require me to add my script to the error page as I have to add new elements such as buttons.
Can I edit the browser’s source code, and then edit the error pages javascripts. Where to start from please? as detailed as possible as this is my first experience in modifying FF code.
Or otherwise, can I edit the error pages using other ways like extension? but I need to replace the pages with my pages before they appear. How to do this especially that the URL does not show the error page name or any thing. Just the website URL.