Be careful with old versions!
These versions are displayed for reference and testing purposes. You should always use the latest version of an add-on.
Additionally, one can use the URL or address bar. Simply assign this search engine a keyword by clicking on the select engine arrow on the left of the search box and going to "Manage Search Engines." Then assign a keyword by selecting the Engine "Enter Pat Num" and then clicking on "Edit Keyword." Enter the desired keyword (e.g. "p") and save.
Now you can type the assigned keyword followed by a space and the patent number and you will automatically go to the patent at the USPTO. For example, if you assigned "p" as the keyword if you type "p 7123456" into the address bar, once you hit enter you will go to US Pat. No. 7,123,456 at the USPTO.
Please also check out the PatentlyUseful for USPTO addon which makes viewing and working with a patent easier. It is located here: https://addons.mozilla.org/en-US/firefox/addon/11317