Problems with Facebook Purity for Safari in El Capitan

Hi there

I used to be able to run Facebook Purity on FB with no problem with Snow Leopard on my old computer. But since upgrading to El Capitan (different computer) I can no longer see FB Purity in my menu bar on Facebook. For me the best features of FB Purity were having the 'chat window' open fully automatically for 'messages' without having to manually do it each time and also seeing when you'd been deleted by friends. I tried to install it again but there were various things about greasemonkey and tamper monkey. I clicked on that part and then something downloaded. Then I tried installing but just got a window of text open. Now what? No Facebook Purity and a complicated list of how to resolve it, one of which was to disable greasemonkey or tamper monkey in Safari extensions but when I go there there's no mention of them.

Thanks in advance