Browser blocking was accidentally disabled by a recent upgrade and several people have reported the bug. I've just deployed a fix for this. As part of the change I rewrote a lot of the internals and also added a lot of tests to ensure that future deployments go more smoothly.
If you notice any issues with browser blocking or anything else, please let me know.
If you want to enable browser blocking, you can do so via the links on your "Account Options" page, which can be found under the "Options" menu.