Removing drip pan on a Frigidaire FFTR1821QS
From the manual for that fridge:


  Drain pan (not removable) You may hear
  water dripping into the drain pan during
  the defrost cycle.


This seems pretty odd to me, but that's what it says.  The toe grill on the front of the fridge does come off to possibly provide a way to see into the drip pan.  I would try to spray or pour (with funnel and tube) some water with a mold cleaner/killer diluted in it.  I would probably try something more gentle than bleach as to not harm the plastic of the tray.

Of course, before any of this, pull out the fridge and make sure it's not something spilled behind the unit or under it.

P.S.  Anything is removable... "not removable" just means it's not going to be easy.  If you're convinced the tray stinks and really needs cleaning, it will come out, but the fridge might need to be on its side.  Possibly not recommended depending on your experience level, but hopefully it doesn't come to that.