After installing Map+ and restarting Firefox, click on the "Tools" menu and select "Add-ons." A dialog box will appear with a list of installed Add-ons, click on the Map+ entry and push the "Options" button. A new dialog box will appear. Here you can enter your address and adjust the size of Map+. Push the "OK" button and close the Add-ons dialog. You're all done!
As an example, highlight the following address: 1600 Amphitheatre Parkway Mountain View, CA 94043 (hold down the left mouse button and drag across the entire address). Now right click on this highlighted text and you'll see a menu entry that says "View map of 1600 Amphtitheatre...". Click on that menu item and a popup with a map of that location should appear. If you want to get directions to that location, simply click on the little bubble icon that appears on the map and click the "To here" link. A new tab will open with the Yahoo driving directions.
A video of how to install and use the Map+ add-on can be found on YouTube: http://www.youtube.com/watch?v=gLIVKbYcH4Q.