Firefox support? Patch incoming.

[jclark-dot-org]

With the release of Firefox Quantum (57.0), I'm trying it out for the first time in years, and finding that some of my favorite plugins aren't on FFX. I'm hoping you'll release this one as a Firefox addon.

Mozilla provides instructions for porting a Chrome extension to Firefox, which I've been reviewing. You'll need to handle the "Submit your add-on to AMO for signing and distribution" step, but I've been working through the other steps. I've only found one minor issue: manifest.json uses the older "option_page" instead of the newer "option_ui"; Firefox only supports "option_ui"; Google supports both since v40 (Jan 2015). I'm going to send a small PR to update the manifest to the new style.