As a personal preference, I like my browser (and most of my applications) to be a single process.
Honest question: Why is this your preference? What difference does it make whether an app is using 1 process or 10 behind the scenes?
There are tradeoffs. If all of the evergreen browsers are headed this route, so be it, I just want to know.
Sandboxing is easier with more processes, at least if the processes are split up to make this easier. One of the reasons for e10s is to allow for sandboxing, so it is now easier to apply sandboxing rules to Firefox.
anecdotally I've found there is overhead for each tab process (of which I may have a ton of very tiny ones in my tree tabs
This is understood and one reason why Mozilla is conservative here.
The plan is to make it the default in Firefox 51 on desktop, assuming the staged roll out is successful.
Also, the current approach will not considerably increase resource usage for heavy tab users as FF will only run one content process shared across all tabs.