The beacons window allows you to locate certain types of objects, by identifying them visually on your screen. The window is activated via the top menu → World → Show More → Beacons, or by pressing Ctrl-Alt-Shift-N.
The slider underneath controls how wide the beacon lines will be. (It does not affect highlight mode.)
The check boxes below control what kinds of objects will be indicated:
You may select one or more of these to be highlighted on screen.
Beacons are handy for finding lost or “unknown” objects, or “griefer” prims.
If you wish to search for items by name or description, try Area Search.
See this page for documentation on Firestorm 4.6.9 (42969) and earlier.