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Zen browser had a backdoor enabled by default
(github.com)
This is a most excellent place for technology news and articles.
I'm not saying Librewolf is insecure, I'm just saying its a bit less secure. They generally do a good job keeping up to date, but there can be delays if an update conflicts with their changes.
Librewolf is not just a Firefox config. You can look at the repo and see a number of patches. Without a paid security team to review these patches with every update, it is less secure.
I'm not saying not to use Librewolf, the likelihood of a zero day specifically targeting it and effecting a significant number of users is very unlikely, simply based off of the size of its userbase compared to more mainstream browsers.