works perfectly! could use more settings (i.e. per-window cycling instead of only the selected window) and a custom icon, but still does exactly what it is advertised to do

Works as expected on Firefox 98.0.2

For convenience, I'd like the toolbar icon to offer a simple pop-up for adjustment of the interval.

Take a little inspiration from Fresh Flower, maybe – https://addons.mozilla.org/addon/fresh-flower/