1. Click the link.
Click the "Install now" link below, click install in the Firefox dialog and reset Firefox.
2. Enable API support from ma.gnolia.
Go to your account in ma.gnolia. In the main menu select "you"->"your profile". Go to the advanced tab and select enable API support.
3. Set your username.
In Firefox, go to tools->add-ons-> magnolia options.
Now you have two options:
3.1 In the first tab set the username and password (the ones used for your ma.gnolia.com login) and click "Save". If the username and password are correct just close the window. If you have a ma.gnolia user&pass saved in Firefox password manager, the dropdown list (in magnolia options window) will have that user and you could simply select it and hit save.
3.2 If you have an OpenID attached to your ma.gnolia.com account use the second tab.
4. Drop the button.
This stage is optional. If you want a button in Firefox main toolbar just hover the mouse near any toolbar, right click, select "Customize", drag and drop magnolia's button wherever you like.
If you don't want the button, you can use the right-click menu and choose "Mark it with magnolia".