The first issue is a browser limitation. As far as I can tell, Firefox zoom settings are all per-origin, rather than per-tab. There's not any way to have two different windows with different zoom levels looking at the same website. I can get rid of the zoom loop by not updating the zoom in that situation (which I think I will do), but that means one of the windows will always be zoomed incorrectly and it isn't always clear which one it should be.

The width selector isn't supposed to be the resolution of your screen; it's the width you want your browser window to appear to be to the website. Don't pick 1920 as the width setting, use the default 1280 or one of the options in between. This will make websites be zoomed in on your larger monitor, but they will be perfectly readable. Most websites just have too much white space when you try to view them with 1920, anyway.