When you can't control yourself with social media, to use this app would be helpful, if it wasn't so easy to disable in Firefox's options. You can even unblock the site at will. There is nothing an extension can do with these things, so try out an app. I'm trying FocusMe out, if I find it for free somewhere. It's not a bad extension, it's just too easy to get around it if you have no power over yourself. I hope this gets checked out and helps someone.

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.