You can delete localStorage items one by one using Firebug (a useful web development extension) or Firefox’s developer console.

Firebug Method

1. Open Firebug (click on the tiny bug icon in the lower right)
2. Go to the DOM tab
3. Scroll down to and expand localStorage
4. Right-click the item you wish to delete and press Delete Property

Developer Console Method

You can enter these commands into the console:

localStorage; // click arrow to view object's properties
localStorage.removeItem("foo"); 
localStorage.clear(); // remove all of localStorage's properties

Storage Inspector Method

Firefox now has a built in storage inspector, which you may need to manually enable. See rahilwazir’s answer below.