Since I plugged this Firefox add-on here, I thought I should update this thread. It may be leaning toward off-topic, though.
Favicon Picker 3 does work great, and I have added icons for my fave's that didn't have any and replaced one's that I didn't care for. However, I recently installed Firefox 4. Favicon Picker 3 does not work with F4 and the developer finny08 will not release an updated version.
His reasons for not updating are here: Meet finny08
I like F4 enough that I do not want to reinstall an older version. I'll have to wait until someone releases a similar add-on.
-Jim