I'm not sure why you linked to this irrelevant 3 week old issue while referring to something that was fixed a year ago. Referring to it as a backdoor also implies that it was malicious, when it was simply incompetence. Have there been any security issues since? (Not trying to imply that not having any would make it safe, just wondering).
Zen is an amateur hobbyist project, expecting it to be something else is silly. It isn't backed by a company, so you take on these risks when you use the project. The same thing goes for all community run browser forks, and unfortunately, using upstream browsers will 100% be more secure. If you don't want to take those risks, just use Firefox (preferably hardened).
Security costs money, open source browser forks generally don't have much of that.
Edit: I'm not trying to shit on this browser, or even say that nobody should use it. Be aware of your attack surface and know what risks you're taking on when using any piece of software. I'm probably still going to play around with Zen, but I probably won't be doing my banking on it.