If you go to the chrome's inspector, there is a "fit" button right at the top which is very handy in responsive design. It tries to fit the viewport in chrome with any screen resolution you set. Does firefox have a feature like this? And how can I use it?
No, we currently don't have this feature in Firefox's Responsive Design mode. It seems nice to have on a small screen or for a large device! I've filed a bug to add it.
We'll be refreshing Responsive Design later this year, so I hope we'll be able to include this feature then.