What do you mean by blocking a specific website?

• If you mean protecting you against some website tracking you online, you can use Firefox Tracking Protection, which will be enabled by default in Private Browsing.

• If you want to prevent yourself from visiting certain websites you could try this add-on.

Thx Gert,

I am already using Minimal Site Block add ons, but the blocked sites can still be accessed in private tabs. I understand, the whole idea of private browsing is for non tracking, but I was wondering if it is possible still to block sites.

If the add-on doesn't block websites in Private Browsing, then maybe you could contact the add-on developer to add this functionality. Otherwise, I don't know of another way to block websites. Sorry.