Firefox DevTools has now a network throttling tool to simulate slow connections, so you can get a rough idea of the user experience under different connection types (bug #1283453).
This is extremely important when building responsive sites, which are meant to perform well on many different devices and scenarios.
To access this tool, make sure you’re running an updated version of Nightly, then open the Responsive Design Mode tool via Tools → Web Developer → Responsive Design Mode. You can then select the desired simulated speed with the dropdown menu to the right of “Responsive Design Mode”, and reload the page to see the effect of the changes.
And if you want to find out what is being loaded, use this tool in combination with the Network inspector: