Eg. Goto Google. Keep the cursor in the text box. Right Click -> Show Hidden Fields. It will show up the hidden fields for that page.
This addon would become very popular if two screenshots are put for it on how to use. First one, to keep the cursor on any form field and right click showing the option "Show Hidden Fields" and the second one which is already present as screenshot.
I have Firefox 3 and it does work for me. This addon will show you the hidden fields on a page in a poorly designed window that pops up over the current page. To get the window to pop up you must find a control (I used a text box) and right click on it. In the context menu, it says "show hidden fields." Click on that and it gives a comprehensive list of all the hidden fields on the page and their values. If there are no visible text boxes, I don't think you can access the hidden fields window with this addon. Once the window is open, you can change them if you want allowing you to test your page. Interesting usage may be for security and robustness testing. Hidden fields are public and can be changed on a user's whim. This may cause unexpected problems for a website developer.
When writing macros or scripts to simulate queries which are usually submitted through html forms, the visible fields are easy to simulate because... well they are visible. Some queries will not work without simulating the hidden fields. Discovering these hidden fields requires some sort of research. UnHide Fields aids in this research.
The majorsuggestions I have for making UnHide Fields more comprehensive and useful are:
1. Display all form fields, method, action, and all input types. This would enhance simulating changes to visible fields without recourse to other tools. Also, it would facilitate writing macros and scripts which need the additional information. 2. Put title bar on the display so that it can be moved.
By displaying all form fields and making the display moveable, UnHide Fields would become a one stop tool for the form information I use for my form simulating sripts and maros.