In the past hitting ESC caused Firefox to halt all functions. It was very useful in situations where lag/poor site design caused clicks to perform unwanted actions. It was also VERY useful when malicious sites start to flood the user with unwanted media, etc.
Now, for some reason, if you click on something, ESC has no effect and the page continues to load no matter what. It is almost as if ESC no longer interrupts the processes, but gets queued along with all the other actions being sent from the website.