I’ve been using Kitten Block for years, since I don’t really need the blood pressure spike caused by accidentally following links to certain UK newspapers. Unfortunately it hasn’t been ported to Firefox 57. I tried emailing the author a couple of months ago, but my email bounced.
However, if your primary goal is just to block the websites in question
rather than seeing kitten pictures as such (let’s face it, the internet is
not short of alternative sources of kitten pictures), then it’s easy to do
After installing the extension if necessary, go to Tools → Add-ons →
Extensions → uBlock Origin → Preferences → My filters, and add
www.express.co.uk, each on its own line. (Of
course you can easily add more if you like.) Voilà: instant tranquility.
Incidentally, this also works fine on Android. The fact that it was easy to install a good ad blocker without having to mess about with a rooted device or strange proxy settings was the main reason I switched to Firefox on my phone.