I'm somewhat disappointed that there is no Firefox version. However, given that the author most likely uses his extension, I can see what might have happened.
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/Web...
To your point, the Orion webkit-based browser for MacOS supports both:
AFAIK, the only webkit browser to do so.
My only complaint is the way it does this thing when you're out of time and it redirects you (either to their block page or a custom page you set up). Not sure how it does it but the original url you tried to reach doesn't get saved in your history. If I try to manually disable the extension it'll also automatically close the page with the leechblock message so it's gone forever. Kind of annoying if it's a link I'd like to revisit after work
Other than it's pretty neat