privacy.resistFingerprinting breaks a lot more than just themes. Many of the weird problems reported in Firefox (and forks) are just from enabling it.

It has some pros but also TONNES of cons. Everything from a completely blank page to wrong timestamps to poor textures and so much more. Sometimes you will be flagged as a bot and prompted with literally infinite puzzles, thus effectively banning you from a website.

Some of these problems get fixed but new ones also get born. I personally use it but I also expect breakage and worse performance.

You shouldn’t give out your real email address. I instead use randomly generated email addresses that forwards emails to my real one. I can easily deactivate any forwarding email address and therefore stop any unwanted emails.

It works really well and also has other benefits like for example knowing who sold your information if you start receiving spam.

You also gain a bit of privacy and security from being a tiny bit harder to track and your credentials being less valuable in the case that the company you gave the random address to has a data breach.

This video explains the concept very well.