As someone who has written several extensions across all the different browsers (IE COM Interop FTW!!!) over the last few years, Chrome's dev experience is by far the best in my humble opinion.
Firefox's experience was hamstrung by having to learn XUL and a new and different set of tools and way of doing things. Their approval process and packaging scheme was surprisingly not as streamlined as I would have expected from them either. Chrome's experience is fairly standard and intuitive HTML, JS, and JSON interactions. In most cases it really shouldn't take too long for someone to port or migrate an old Firefox plugin to the new format.
Firefox's experience was hamstrung by having to learn XUL and a new and different set of tools and way of doing things. Their approval process and packaging scheme was surprisingly not as streamlined as I would have expected from them either. Chrome's experience is fairly standard and intuitive HTML, JS, and JSON interactions. In most cases it really shouldn't take too long for someone to port or migrate an old Firefox plugin to the new format.