I love GOTG the comic book. Its easily my favorite series of the 2000's. That said I hope they don't make a movie out of it because it would be a disaster. The special effects would require a massive budget and in case you haven't noticed the comic book movie market outside of Avengers has been rather soft the last few years. Anyone remember Green Lantern flopping last year? Ghost Rider fizzling this year (though with international numbers it did make money). Captain America and Thor taking in less domestically during their entire runs than Avengers did open weekend.
And comparison to Serenity / Firefly. . . . . .ugggh. Don't get me wrong, I love Serenity and it happens to be my favorite movie of all time. But it BOMBED in the movie theater. Truly and completely BOMBED. Josh Whedon made it on a shoe string budget of $39 million and it didn't quite that much back with international #'s thrown in. Now cut off the 30% theaters get and add in the marketing budget and you've got a movie that lost somewhere in the range of 10-15 million for its studio. Thats not the comparison you want to make for a GOTG movie.
Lastly comic book fans are accepting of a certain amount of silliness. We can look at two silly characters like Cosmo and Rocket Raccoon and enjoy them and maybe even find them awesome like I do. General audiences won't be nearly as accepting.