A point Jessica made in a comment below made me stop and think. She said: "The whole story is theoretically being "told" (though I think that's just a narrative construct, since Firmin has no one to tell this to and no way to write it down) as he's finally dying of old age, hallucinating while Scollay Square falls down around him." I hadn't thought of this while I read the book. I accepted the fact that I was reading about a sentient rat as "real" from the outset. It wasn't until later that I looked at the Chuang Tzu quote that I thought maybe he wasn't a rat. But now having thought that and going back and reading the last chapter, I wonder if I read the end incorrectly.
Jessica mentions that Firmin is narrating his life story, but to whom? Who's he telling it to and how if he is a sentient, but non-speaking rat? He doesn't die at the end of the chapter, but you're led to believe he's in the process. But what if he's not a rat? What if he's a man who thinks he's a rat protesting the changes going on around him? Perhaps it's just his rat persona dying. And perhaps he then is free to narrate his life as a rat to someone later on. I'm not saying this is what I believe. I'm saying that the quote made me radically change my perception of the book as a simple story about a rat and that's why I enjoyed the book so much more.