As soon as I install/enable this extension, Firefox slows down exponentially. If I let it run for more than a minute or two I end up locked out of my desktop environment and I'm forced to hard restart my machine.

Tested on Firefox 73.0.1, Kubuntu 18.04, KDE/Plasma 5.12.9

I followed the very clear instructions to make sure I wasn't missing anything, and I had set up the add on correctly. Still, it straight up blocked a site, although I had set it up to block me after 30 minutes. It's a very easy to use system, and I am 100% certain that I didn't tell the add on to block me- I set it up to block me after 30 min in a day. I hadn't used the website at all yet today. The grayscale filter also produced an intolerable level of lag that resolved when I removed the add-on. I installed a similar add on that was able to allow me limited access to the target website.

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.