Hello John Corliss, correct, when installing the add-on, it retrieves once each favicon directly on their page and caches them. This is because FF does not give access to its own DB of favicons :-(
Cf. https://bugzilla.mozilla.org/show_bug.cgi?id=1315616

You can let it go and then it stops when all favicons are fetched (duration of the phase will obviously depend on the number of bookmark items you have), or as you did you stop fetching and then miss the little favicons. This will get better when FF fixes the above bug / adds the function.

Thank you, aaFn.