I only focus on Chrome so far and haven't do much survey to the entension sysytem of Firefox. Integrate with Firefox could be a next challenge to move on
Another reply touched on the possible differences in extending devtools, I can't speak to that.
I've built several extensions now that worked seamlessly in both Firefox and Chrome. Firefox exposes a `chrome` object that very nearly matches the Chrome extension API and a `browser` object that goes beyond it (e.g. Promise-based async handling instead of callbacks, some additional functionality).
The only incompatibility between the two that I've specifically come across is that Firefox doesn't allow background pages to be specified as persistent. Otherwise if you're doing fairly standard stuff with content scripts, background scripts, browser actions and page actions everything should Just Work.
