Which Linux distributions configure any browsers (reliably, by default, without me having to know extra stuff) so that it cannot read or write files from any directories the user has access to (allowing, say, only /tmp and ~/Downloads and ~/.cache or the like), and so that if it makes inappropriate system calls (any except those on a whitelist) they fail?

As a longtime (and grateful) Debian user, I remain impressed with OpenBSD's relative lack of privilege escalation bugs in the base system, and "only 2 remote holes in the default install [since about 1996]": we can have different views of course but their constant auditing and general approach to correctness and security over adding features does make me feel better. :) Not trying to start a flamewar though (that would be bad; I fear I might be talking like a fanboy now...).

I appreciate your comment.

I certainly don't meant to claim that Linux is more secure than OpenBSD. OpenBSD's security is better in several respects. I'm just trying to keep this about the browsers themselves.

I'm glad to know about about:preferences as a tab for future ref.; thanks for the info!