This addon does work, I know that it seems sketchy that you need to download a file to make it work but bear with me.
First install the addon and click the thumbnail icon (or the shortcut ctrl+shift+f7) download the extra file. Unpack it and copy the files to your firefox addon folder.
To find your firefox addon folder go to about:support in your firefox browser and there should be an area called profile folder, click the button next to it labelled "open folder". Now copy the install, uninstall and aot files into that folder.
Now run the install file and your good to go. If you press the thumbnail icon (or shortcut keys) the firefox window will stay on top. Press the key again to stop the firefox window from being on top.
This addon is great, it only got 4 stars because of the extra digging I had to do to get it to work (I'm a windows user by the way). If you could find a way to fully automate the process this addon would be a perfect 5.