firefox-config VS thunderbird-user.js

I have used many of these forks and yes, I would rather say they are really good out of the box in terms of privacy and also hardening. However, whenever they make some changes after firefox updates, they become somewhat late in terms of their update so you'll be missing out on that part, rest is great. What I use is the original Firefox, along with ublock and some custom configs along with arkenfox and a customized overrides file which takes care of maximum performance and usability. I also maintain a repository at https://github.com/prirai/firefox-config so make sure to check it out. I'll be updating it soon.

There's this GitHub repo called thunderbird-user.js which is similar to (and also inspired from) the arkenfox project (for Firefox security and privacy tweaks). I suggest you check that out :)

I'd also consider taking a look at https://github.com/HorlogeSkynet/thunderbird-user.js I ported it from Firefox to Thunderbird, compared/looked at source. I handed over maintainer ship to someone else though and I don't personally use Thunderbird anymore. Nowadays I use a text client https://neomutt.org with a setup very similar to this.