Click on "Livejournal Interest" (for interests) and "Livejournal User Info Search" (for usernames).
This will install those two features as Firefox search engines.
To use: go to the search engine box on Firefox -- it's in the upper right-hand corner -- and select either the LJ pencil (for interests) or the head-and-shoulders person icon (for usernames). Type it in, hit return, and watch the results pop up just like they're supposed to.
Posted here by suggestion of foxfirefey on the latest news post. I didn't make either of these engines; I just found them through the magic of Mycroft.