Although I love Farscape, I don't really equate it with Firefly. Their styles are very, very different. As for worrying about strange aliens, don't. Firefly is a 'dead' universe similar to nuBSG's--there aren't any aliens to speak of.
The tone may be a little off-putting at first, but you'll get used to it. I rather like the series, though I could go without some of the vociferous fans. I dislike the movie, but your mileage may vary. Just make sure you watch it in the DVD order.