Those “personal pet projects” are why Google and FireFox exist as many pieces of their projects often rely on open source components often maintained by a single person.

Those are a different kind of pet projects, like some small random math library developed by a guy in Nebraska that a big software stack depends on (there's a relevant xkcd about it somewhere). The thing is, if support for such a project stops, the Microsofts, Googles and Firefoxes of the world are able to take over support, pay for it to be supported, or work around it in another way. Plus they are usually careful about which dependency they introduce, if something isn't governed properly or does not have wide community support... it's unlikely to be included.

Taking on a whole browser as a pet project is something entirely different. Browsers are huge and complex. You're basically betting that mr-cheffy will be able to keep up with all the changes, like security updates, feature updates and bugfixes, that upstream Firefox produces, and that he will be able to keep his own part of the codebase secure, and that he won't get burned out or bored with the project in one or two years.

For these reasons, I will never put all my eggs into the basket of some 1-man browser project, sorry.

These pet projects also strip telemetry and respect your privacy.

Turning off telemetry is just a few clicks, or about:config flags in Firefox anyway. And "respect your privacy" is just meaningless buzzword bingo. If you go to facebook or google in zenbrowser, your data is harvested just like everyone else's. Privacy is a process not a product (browser).