As everyone mentioned, for a 50th birthday (or any adult birthday), I would truly not recommend any restaurant inside MK unless your birthday gal/guy is a die-hard Disney fan. (And if that's the case, then he/she would normally tell you where he/she wants to go.) That being said, I would avoid Be Our Guest and Cinderella's Castle unless birthday guest is a HUGE Disney-theme person. As previous posters mentioned, Be Our Guest is TERRIBLE for pretty much any type of socialization. I, for one, was so overwhelmed by everything going on (the theming, but moreso how loud it was, busy with people traipsing past the table ALL the time, bad food <-- altho this was when it first opened & I think it's since improved based on more reviews). I haven't been to Cinderella's castle for 25+ years, so can't comment on that. But I do think, from pics, that it's still super kid-focused. For a 50th bday, I don't know if that's the right place.
As previous posters mentioned, Cali Grill is INCREDIBLE. It's my fave special occassion restaurant.Tell the host when you check in that you're celebrating a bday (and also have it noted on your ressie). Ask for a window table. Time the ressie for 45 min to an hour before fireworks (if possible) if you want to see fireworks. You can either enjoy them at the table (they pipe the music in), or go out to the observation decks (also pipe music out there). Super easy walk from MK, or you can take the boat. It's a much lovelier, special experience for any special occassion, especially a 50th bday! Enjoy.