I use Firefox personally and originally made the thing for myself. I added Chrome support because Chrome is much more popular (not far from ten times more popular these days :() and people I would like to use this, eg friends and family, mostly use Chrome.
I couldn't ask them to switch browsers for my little side project. I have to co-operate with Google's bureaucracy. For what it's worth, so far it seems like Mozilla is not exactly streets ahead, but at least they didn't charge me and they seem to be fairer and more helpful to extension developers (they have a "self-distribution" mode with relaxed oversight I used while in private alpha, and their tools and docs are better).