I have a portable window unit (I can't say what brand or model it is right now as I am not where it is right now), but I can say that it is an older one, perhaps 15 to 20 years old. This year I noticed that the black foam filter in the front of the unit looked dusty, so I removed it to clean it. It was so old and rotten that it tore diagonally about 2/3 of the way from the top. I pieced it back together as best I could and put it back in the unit, but now the condensation is running off the window sill and pooling on the floor under the window. I have installed it as I always have previously, and there's never been a problem with leaking before. Could the tear in the filter be causing this? Any other suggestions?

At that age the unit needs to be removed/disassembled and cleaned. When done with that replace the filter and reinstall with a slight tilt to the rear/outside for good drainage.