Great addon. However there is an issue on Firefox for Android with the time limit: the countdown keeps ticking when I switch to another app. (when the countdown reaches 0:00 the current page stays unlocked => my bad, it's a setting)
Thanks for the review. The Firefox for Android issue is a known issue; I'm working on a fix. Not sure about the second issue. Make sure you have the setting "Immediately block pages" selected.
