The webdeveloper extension is not only useful to webdevelopers.
Show password displays the password in password-fields. So you can see what you type. (If there is nobody standing behind you, there is no need to hide characters behind asterisks.)
If you use the firefox default settings and you search google and use a link provided by google to visit the found page, the google search-parameters (e. g. the keywords) are sent to the computer hosting the visited webpage. The same applies to link-collections.
Disable referers protects your privacy. The webpage you visit is not informed from which webpage you come from.
You can also disable meta-redirects, Java, JavaScript, the popup-blocker, css, …
You can convert post-forms to get-forms. Many websites supports get-parameters although they use posts. Int his case it is easy to create an URL to a webpage that can only be reached by submitting a form.
Most of the many other features only make sense to web-developers I think.