Hi koodawg,
The set of thumbnails is refreshed only on Firefox startup, so if you
had Firefox opened for a long time there would be no changes until you
re-opened it again. Also, algorithm for deciding which sites are
chosen to be shown as most frequent depends on long term and short
term usage - this list should be similar to the most frequently
visited sites of the URL Bar (awesome bar); but it isn't always.
For the next release it is planned to update the set of thumbnails
more frequently - thus eliminating the first problem. Also the set
will be the same as the one from URL bar to eliminate confusion.
While you might not be interested in the answer any more, this is for
the benefit of other users who might have the same question.
Cheers,
Mihailo