It depends on the problem domain. Can you guess ahead of time how many child nodes that div will have, how many tabs the user will open, or how many functions this javascript file will create?
The classes in Firefox that he's talking about all start at some size larger than zero, either by default or by specifying a size when you construct them. For instance, if you know the string you're working with can live on the stack (it won't have to stick around in the DOM, or be sent to another thread, etc), you can use a string class that starts out as ~32 bytes on the stack and only allocates on the heap if the string grows larger than that. This saves an immense amount of allocation.
The classes in Firefox that he's talking about all start at some size larger than zero, either by default or by specifying a size when you construct them. For instance, if you know the string you're working with can live on the stack (it won't have to stick around in the DOM, or be sent to another thread, etc), you can use a string class that starts out as ~32 bytes on the stack and only allocates on the heap if the string grows larger than that. This saves an immense amount of allocation.