Browser & Addon:
Well, Dissenter has been an add-on as well and not just a browser. I use Dissenter every day and it works very well. But they had a FireFox add-on prior to launching their browser which is a fork of Brave like you said.
Currently
I don't currently know if the add-on still works on Firefox to date but it at least did work. I also don't know if that add-on worked on other web browsers or not. I will let you know if I get around to trying Alt Yes. I also use Ubuntu.
Alt Yes
Dissenter let's you share things to Gab. It sounds like Alt Yes let's you share stuff to Steem. So, that is why I compared it to Alt Yes. But, at the same time, Alt Yes might be more universal like you said, it may be working on more browsers. I don't know if that is true but it might be true.