Yes that was possible and I always used it to subscribe to TTRSS. One day after an update I saw that they removed the button (which used https://developer.mozilla.org/en-US/docs/Web/API/Navigator/r...) with which you could subscribe in Firefox that your custom TTRSS url should be added to the list off the ways to subscribe and I wrote a plugin to read it https://github.com/jeena/tt-rss-plugins/blob/master/jp_fxsub...

That worked for a short time until Firefox removed that API. Now I need to install this addon to be able to do it https://addons.mozilla.org/en-US/firefox/addon/awesome-rss/

It's basically impossible for people to find out how to subscribe to a RSS feed nowadays.