First, i have seen Outlaw Star. I have yet to Finish Firefly (i am on episode 9) and I have seen the film.
BTW here is the link to the similarities:
Second, Did Joss Wesdon rip off the idea from Outlaw Star, i dont know and I hope not.
Just read Wikipedia, Firefly, and that should help out.
But the similarities are too much in common in my opinion.

Bottom line: they both have a great sense of adventure, comedy, and western motif. i have enjoyed the both of them and both are great to watch.

so watch both of them