When you click the Firefox extension link you'll get this dialog:
Click the OK button, then you'll see this:
Select install when the button becomes available. The delay is due to Firefox's built-in scanning of extensions to ensure they won't create problems with your browser. You'll then you'll be prompted to re-start Firefox. You can do this any time but the LaterMaybe extension won't be available until the next start-up.
When you next start, you'll see this dialog:
This confirms that you've installed LaterMaybe. You can close this dialog box since there are no options to set. You can remove LaterMaybe any time by going to the Tools/Add-ons menu. An alternative to this installation method is to download the file (latermaybe.xpi) then drag it onto an open Firefox window.
As a browser extension, this tool is very minimal. All it does is add an item - "LaterMaybe" to Firefox's context pop-up menu, and when this is selected it executes a Javascript call to open a window at latermaybe.com, passing as paramers the link you clicked, along with its description, or the actual page and title if you clicked a blank area of the page.