The default icon size (Icon Dimension in settings) is set to 64 css pixels, but every firefox version I tested it on (nightly,release,esr on linux;release on windows 8.1 vm), managed to downscale it to the correct size of (most of the time) ~16 css px[1]

Setting Icon Dimension to 16 px should have definitely fixed it in any case.

If that does work, I might set it as the default, since currently only firefox is supported anyways

[1]: https://developer.mozilla.org/en-US/Add-ons/WebExtensions/manifest.json/browser_action#Choosing_icon_sizes