This is wishful thinking. How confident are you that you'll spot a well planted backdoor? Hint: the more capable you are, the less confident you should be.
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.