Yes. The first line of the article ("a SUID program that reduces the risk of security breaches") should be enough to raise a quizical eyebrow from security-minded users but overwhelming positive commentary I've seen elsewhere (lwn?) has not mentioned this.
Some of the obvious holes (eg "mount a tmpdir on any mount point") have now apparently been closed but I haven't been back to it for a while. Early releases did everything euid==0... It certainly needs more pairs of eyes.
OTOH the largest attack surface presented by a typical single-user Linux box is probably the browser and not an unprivileged local user looking for escalations (assuming: firejail drops privs perfectly and an attacker cannot trigger a vulnerability in SUID firejail from the application that it invoked). So could be that's a reasonable trade off for some folks.
> like the idea though
I agree, the feature set is nice but it needs some more basic TLC. "Security products do not necessarily make systems more secure" </rant>