If you're that paranoid, it's actually not very hard to audit the code in this case. Unless you go so far as to not trust the whole stack it's running on, but i

If you're that paranoid, it's actually not very hard to audit the code in this case. Unless you go so far as to not trust the whole stack it's running on, but in that case you should stop using your browser entirely. For a sane threat model the confidence in your own audit should be reasonably high.