This argument lacks for the average consumer. Heavily. Do you compile every update of Firefox yourself (and this assumes that you have read every line of code that changed during updates)? Especially when on Windows or macOS, you just download the version that is distributed by mozilla.org on their website. There is no guarantee that they're using the actual sources to build. They could just as well add in a little patch that does some nasty things.

But they won't, because they have a reputation to loose. So does Vivaldi.