Using your mouse or touchpad, right-click on the Open Source bookmark and then left-click on Properties to bring up the dialogue box shown above.
In the Keyword field, enter "oss" and then left-click on the Save button. This will tell Firefox that you want to associate the keyword, "oss," with this bookmark. You can type any text into this field, but if you enter a keyword that's already tied to another bookmark, Firefox will only use the new association moving forward.
Browse away from this page so that we can test our new keyword in the next step.