SUID by itself is not a security liability. Typically, programs using SUID quickly drop their additional privileges during startup to avoid the scenario you're describing. So the vulnerability in firejail would have to be early during startup before any of that.

Still, it can happen of course, particularly with memory-unsafe languages. Conversely without root privileges you have to sacrifice strength of your sandbox, widening the vulnerability window in the app you were trying to constrain to begin with.

Meanwhile bubblewrap relies on user namespaces, which come with their own set of security problems. They are disabled by default in many distributions for that reason.

Yeah, it runs with escalated privs. People wager that SUID binaries are extremely risky and this seems totally reasonable to me! I don't know where to find a really good security analysis of the risks, however. Firejail is most likely not bullet-proof, although not soft by any stretch of the imagination and I don't know how often and how many security researchers are profiling this tool.

My gut sense is that flatpak gets much more scrutiny since it ships, and firejail is typically not shipped, but another package as far as I can tell (maybe in some specialized distros?).