This was one of the feature that made me get my first Mac back in 2002. You could easily tell apps that didn't use OS standard controls (e.g., Firefox) because

This was one of the feature that made me get my first Mac back in 2002. You could easily tell apps that didn't use OS standard controls (e.g., Firefox) because they didn't support the dictionary lookup. At the time it was ctrl+cmd+D while hovering over a word with the mouse cursor (you can still trigger it this way now).