Note that the further I went down the sandboxing rabbit-hole, the more questions it raised about whether it's more or actually less secure. The main problem is that in order to work, these tools often use a setuid binary, which actually has more permissions than most users. So in theory if a sandboxed app finds an exploit in the sandboxing program (like firejail) that you're running inside, you could actually be worse off than it breaking out of whatever program you're sandboxing in the first place.

I think in this case though where you're more concerned about the very real problem of websites accessing localhost, it probably outweighs the maybe of a firejail exploit.