I'm a big fan of OpenBox, but out of habit I use a couple of Gnome apps. Most notably Gnome Terminal. For some reason, when I clicked URLs in the terminal they opened in Epiphany. Firefox is still my main browser, so I wanted to change that. I solved it like this:
- sudo pacman -Sy gconf-editor
- Run gconf-editor , edit the /desktop/gnome/url-handlers/http(s) keys to your liking. In my case "firefox %s".