We have an extension that we have ported to Firefox, that works in Chrome, but also web and mobile web. So in the CSS there are media queries based on the screen width. A user contacted us with a very weird issue, and after a lot of debugging I was able to see that there was an option called “privacy.resistFingerprinting” that this user had set to “on”.
When debugging the CSS in the extension popup window was that it was hitting media queries that it normally shouldn’t. It turns out that turning this setting to “off” fixed the problem! The CSS was fine and no media queries were hit.
It seems that with the feature on the
screen.width is set to the size of the of extension popup window ~320. When it’s off it is set to the full screen width.
I’d call this a bug in Firefox. How would one go about getting a statement from a Mozilla dev?