My go to developer tool in Firefox and Chrome. Reliable, easy to use, able to inspect element in Responsive design and see how changes will look. Almost perfect (if the inspector could show these changes across all screen sizes, this would give it 10 stars)
As it is now, it's mostly unusable on a screen that's 1280 wide since most of the functionality is hidden offscreen. That's mostly Mozilla's fault, but hopefully the developer will find a way around their incompetence.
This provides a lot of very useful tools, but if it's pinned to the overflow menu, the panel it opens in is too small to use them all. There doesn't seem to be a way to resize the panel or scroll within it.