When I'm composing a comment in Firefox (3.5.5) on Windows or Mac, the "characters left" counter below it counts down. When I hit the limit and continue typing, the number goes negative.
Chrome (22.214.171.124 and 126.96.36.199), on the other hand, will not accept any more characters once the number reaches zero. I have to stop mid-sentence to delete earlier text so there's room to complete what I was typing before.
I prefer Firefox's behavior. It lets me finish my thought and then go back afterward to shorten something from earlier. Furthermore, it tells me how much shortening I need to do. I'd like for Chrome to behave the same.