Definitely a step in the right direction ir order to have the same experience as Opera. There's only 2 things I don't like about this extension: first, WhatsApp needs to log in my account everytime I close and open the side bar, so I can't get notifications when is closed thus forcing me to have the side bar always open, and second is that the side bar doesn't overlay through the browser tab so it moves/minimizes it content.

I'm aware those issues are related to how Firefox sidebars works and with the fact that what this extension does is open WhatsApp web in the sidebar so I suposse that when I have the sidebar closed it act just as if WhatsApp web is closed too so it's impossible to get notifications as it's not working in background. In any way, this extension took a huge effort and it has turned great, it's definitely a must have for Firefox (and it works even better in Nightly's sidebar)!