What I've done is pour in tons of baking soda (sodium bicarbonate) on the floor of the oven, then spray on lots of vinegar. Then I leave it to fizz & sit there overnight, & then in the morning I just wipe it all out, which takes out all the crud. Then I spray it w/vinegar again to wipe it clean.

I've never had a huuuuuuuugely messy oven but this seems to work for regular sorts of mess/usage, including burnt bits that have dripped onto the bottom.