$ sqlite3 places.sqlite 'select rev_host, sum(visit_count), sum(visit_count) * 100.0 / (select sum(visit_count) from moz_places) from moz_places group by rev_host order by sum(visit_count) desc limit 20;' | sed 's/\.|/ /;s/|/ /' | while read rev_host count percent ; do printf '%7d (%6.2f%%) %s\n' $count $percent $(echo "$rev_host" | rev) ; done
  18182 ( 23.25%) www.flickr.com
  14258 ( 18.23%) www.google.com
   4585 (  5.86%) draft.blogger.com
   3652 (  4.67%) mail.google.com
   2994 (  3.83%) appengine.google.com
   2973 (  3.80%) twitter.com
   1809 (  2.31%) localhost
   1683 (  2.15%) code.google.com
   1338 (  1.71%) friendfeed.com
   1175 (  1.50%) dfed
   1033 (  1.32%) groups.google.com
    991 (  1.27%) www.blogger.com
    764 (  0.98%) bbs.archlinux.org
    740 (  0.95%) www.google.com.tw
    658 (  0.84%) brps.appspot.com
    655 (  0.84%) flickr.com
    569 (  0.73%) typewar.com
    559 (  0.71%) www.last.fm
    552 (  0.71%) feedburner.google.com
    521 (  0.67%) www.youtube.com

I really need to quit visiting this page.

You can find places.sqlite in ~/.mozilla/firefox/<profile>/. The counts do not much what Page Info dialog reports, I have no idea what cause the differences. Page Info dialog gives smaller counts.

I don't see any place you can get a list, so I decided to dig into the database of Firefox. It's quite interesting that it has a reverse host field not a host field, the characters of host string in reverse. To get the order back, just pass it to rev.