It might be there is nothing to block. What is Stylus suppose to do to the page?

I don't think that's the case, the add-ons literally don't work. Activating the element picker in uBlock does not display anything, adding custom CSS styles on stylus doesn't affect the page either.

Most (all) Mozilla websites prohibit to inject JavaScript on their web pages via CSP policies (only the website is allowed to inject JavaScript). This makes extensions that use this to stop working. Most extensions that are affected have a remark on their Addons page to notify you. This affect JavaScript bookmarklets as well.

• https://developer.mozilla.org/en/Security/CSP

Does that mean it is impossible to use uBlock and the like on pages with those CSP policies? I wanted to change some elements of the site

If you can't do this with an extension because of CSP then you can use userContent.css with a @-moz-document domain(){} block to restrict the code to a specific domain.

I have found a solution. Sites listed in extensions.webextensions.restrictedDomains at about:config restrict WebExtensions. To solve this, remove the site in question from extensions.webextensions.restrictedDomains .