Does anyone still type "www" ... ever?
I think Firefox just tries several common TLDs when it can't resolve a name, i.e., when you type "ebay", it tries "ebay.com", "ebay.org", "ebay.net", etc. No integration with Google required.
That's why "ebay" gets you to "ebay.com" and "slashdot" gets you to "slashdot.org", but "del.icio" doesn't go to "del.icio.us", and "last" doesn't go to "last.fm" even though those are the #1 Google results for "del.icio" and "last".