Huh, hadn't seen that bit before, thanks for that. Ok, well that is disappointing. I did notice this bit in there too though:

What about security benefits? Even though most of the security-relevant code for Vivaldi browser is in Chromium, there is also some security-relevant code in the UI. If you think that specific security-relevant parts of the UI should be open-sourced to make Vivaldi more trustworthy, let us know, and we’ll consider putting it out as part of our code bundles, so you can check it for yourselves.

It not much consolation, but it's better than nothing. As it stands though, FF still has too many problems for me. I'll have to see how this ad blocking thing shakes out though, might have to revisit my decision then.