Once installed, just right-punch any link, and choose “Create TinyURL for this Page”.
Note that this doesn’t save as a bookmark, it saves in the extension’s saved panel.
To view TinyURLs that you have previously saved, go to Tools \ TinyUrl Creator \ View Saved.
Here you might see the list of saved links, and then copy the TinyURL to your clipboard.
This is extremely useful for links that you oughta use often.
Download TinyUrl Creator from Mozilla Add-ons