List of available operators

:has(...), :has-text(...), :matches-attr(...), :matches-css(...), :matches-css-before(...), :matches-css-after(...), :matches-media(...), :matches-path(...), :matches-prop(...), :min-text-length(n), :not(...), :others(...), :upward(...), :watch-attr(...), :xpath(...).

So the best and cleanest way is probably to rely on chrome.extension.getURL("/") . It can start with "moz", "chrome" or "ms-browser". Unlikely to change in the near future.