Perfect!
I found myself often scrolling through social media and news pages while I was supposed to get work done. They call it procrastinating. This simple yet effective add-on blocks chosen URLs after my set time limit and remind me to focus on my work. Of course there are some workarounds for the block, but this is a solid treshold for work avoiding behaviour.

Its really workin! Thank u!
Guys, use this way for writin sites 4ex - *://*.vk.com/*

Does exactly what it says. Straightforward configuration (from the Add-ons Manager in the menu)

I really like this extension, it works exactly how I want it to. It would be good to have a slightly easier system to add sites to the block list, but I just copy and paste the example and add my own sites. I'd also like to customise the pop up message. I'm finding it is helping with my productivity and that is great.

I always liked this extension before the Quantum update and some of the functionality has changed. Is there a way to remove the push notifications that pop up, telling me time is expiring? Thank you.

The add-ons does the job of blocking websites that will steal time through procrastination. However, it could be a little easier to use as websites have to be entered with specific coding e.g. *://*.facebook.com/* but a fairly easy work around is by adding a new line, pasting the above example and replacing the website address inside the special characters. Once the website has timed out, there isn't a prompt saying that your time has expired, it will just reload an empty new tab (unlike leachblock which will display a message), so it's easy for a novice to mistake this for an error. Other than that, can't complain.