Cool. Is there a non-WebExtensions XUL version for us Pale Moon users?
— How quickly will serious vulnerabilities be patched?
Given the project is so small, I wouldn't trust their future response to be rapid even if they have been in the past. Right now most security fixes come in the form of merging upstream security fixes from Mozilla. As their code base becomes increasingly divergent to the Firefox head, merging in upstream security fixes will become increasingly difficult and increasingly cumbersome.
— How much do you really trust these developers?
I don't think many people realise just how important your browser is when it comes to trust. When you access online banking, your browser is in ultimate control of the entire trust chain. Your browser validates the security certificates. Your browser decides what to do with the passwords you enter. Your browser decides what to show you in the address bar.
— What's wrong with Firefox 67+ anyway?
The one thing everyone seems to cite when they say they use Pale Moon or Waterfox is that they still support the older extension system. The reality is all important extensions were unaffected. And while some genuinely useful extensions were lost in the transition (e.g. DownThemAll) most should be standalone applications anyway.