The distinction of release and beta channel is not what matters, if the userscript version you are using is higher than 2.1, it will be compatible with latest stable Firefox release, and it should perform almost the same level as in chrome. However, popup scripts typically do user agent sniffing and use different strategies to open popups, so it can happen that a popup is blocked in the same page with Chrome but not in Firefox. If it happens, please report it so that we can fix.
The distinction of release and beta channel is not what matters, if the userscript version you are using is higher than 2.1, it will be compatible with latest stable Firefox release, and it should perform almost the same level as in chrome. However, popup scripts typically do user agent sniffing and use different strategies to open popups, so it can happen that a popup is blocked in the same page with Chrome but not in Firefox. If it happens, please report it so that we can fix.