In general, you should use as few as possible.

On FireFox browser, the best privacy extension is uBlock Origin. I wouldn't use any other extension for ad blocking or privacy unless it is for something specific you need that uBlock Origin cannot do on its own. NoScript is another privacy extension that is sometimes recommended.

There may also be some benefit to using the extension for your password manager, as it may help you identify phishing links and prevent you from filling in your password on fake or scam sites. I strongly recommend Bitwarden as a password manager.

Installing extensions is unlikely to help you defend against fingerprinting. If you are concerned about fingerprinting, then you should enable resist.fingerprint in the about:config on FireFox, or use either Tor or Mullvad Browser (which you should not install any extensions on).

can someone explain extension fingerprinting to me? i’ve always heard about it, but to my layman brain it doesn’t make sense that a locally executed modification of css (in the case of dark reader) gives any kind of data to the site host. i guess for ublock it makes more sense since i’m guessing that has to do with blocking specific requests from going out in the first place, or what?

It's probably an exponential function with a negative curve. I imagine what extensions you are installing might matter more