In r3dfox, the zoom level is sometimes distorted when reopening closed tabs. For example, I use 288dpi (300%) in Windows 7, and I added 0.333333333333333333333 to toolkit.zoomManager.zoomValues in about:config. When I set the browser zoom to 33%, window.devicePixelRatio is set to 1 as expected. But after closing and reopening the tab, the zoom gets distorted, and window.devicePixelRatio returns 0.9836065573770492 instead, which according to
Number To Fractions corresponds to the fraction 60÷61. This implies that the zoom got distorted from 1÷3 to 20÷61. The exact same issue occurs in both r3dfox 153.0 (64-bit), Mypal 68.13.1b (32-bit), while Firefox ESR 52.4.1 (32-bit) returns a slightly different value of 0.9836065769195557 (4125545÷4194304, which is probably a single precision float approximation). This indicates that the distortion of zoom values affects a broad range of Firefox based browsers. It should be fixed to preserve the exact zoom level across sessions.
Another issue relating to zoom levels is that when an image is opened in a new tab in the browser, it always defaults to 100% zoom, and clicking on the image will change it to 100% zoom. But in non-96dpi configurations, this causes devicePixelRatio to be other than 1, and makes it no longer a pixel-perfect scale, forcing users to manually change the zoom each time the image is loaded or clicked.