Can cats and foxes mate and produce viable offspring?

Hi, Two years ago, I was surprised to see that one of my neighbours' black cat was quite chummy with a fox. I would have thought the fox would have eaten the cat! But I have seen them together on several occasions since then. And two weeks ago, I saw a very strange animal on the car park next to our block. It was a VERY large cat, at least twice as big as a normal cat. It was not fat, it was big & tall. The colour of its fur was also quite strange: it was like big patches of black and brown, and quite bushy. Could that creature be a mutant, a hybrid between a fox and a cat ? I thought that interbreeding species was genetically impossible, but is it just very unlikely? Thanks for your help, cjc