Copy the below code and paste in the address bar of your browser to view hidden password on that page.
To make it more simple you can add it as a button on Firefox bookmark toolbar as bookmarklet, so when you need to reveal any hidden password simply click the button on the toolbar.
How to add bookmarklet button on Firefox Toolbar
Add password revealer – Drag this link to your bookmarks toolbar
Right-click on Firefox toolbar and choose New Bookmark. Add the above code to the location box of the bookmark and Give any name for the bookmark as you wish, e.g. Show Password. Now, Click on Add and now you may see a new button on the toolbar.
This comes very handy for people who often makes mistake while entering the password or who forgets password. Some bad people out there will definitely be thinking this to be used as to reveal others password.