When I need to find an old backroom thread, I do the following:
  1. Bring up the "find" function in Firefox
  2. Type in word or phrase that I think was in the title of the thread I want
  3. Click back one page at a time through the Backroom, waiting for the "find" field to light up

Usually works. Don't forget that you have to enable the older thread display at the bottom, forget what it's called.