Instead of limiting the time I could spend on the target website, it entirely blocked it and redirected to the alternate website I had selected. I hadn't used the target website at all today, and it wouldn't allow me to use it for the 30 minutes I had set it up for. If you want to just block a website, this is effective for that, but if you're looking to limit the time you spend on a website, you'll have to find something else.

Probably a bug, but this extension slows firefox down to a crawl on Kubuntu 18.04 (up to date as of 02/25/19). Loading URL webpages takes significantly longer.