Found it through nfl.com. Since I didn't know and wouldn't have guessed it, I won't give out the answer. Saw that Tony Franklin and Pete Stoyanovich hold the record for the longest FG by a rookie (59 yards) but not on their first attempt.
Mikey Koenan is the exception! <?XML:NAMESPACE PREFIX = O /><O></O>
Here is the story as reported by the Atlanta Falcons:
And his half-ending, momentum-swinging 58-yard field goal Sunday afternoon against the New England Patriots is the longest in the League this year, four yards better than Neil Rackers' 54-yard three-pointer against Seattle in week 3.<O></O>
To make Koenen's boot even more notable, it was the second longest field goal in Falcons history and the third longest field goal in NFL history by an undrafted player.<O></O>
That, and it was Koenen's first-ever NFL field goal attempt. Not to mention the longest field goal he's made. As in, ever.