Add-Ons I have installed are:

• uBlock Origin
• uMatrix (sadly deprecated)
• Privacy Badger, shows/blocks trackers that made it through uBlock/uMatrix (which are not many if at all)
• Decentraleyes, which caches libraries from CDNs so you don't connect to these central servers again, disclosing your usage pattern
• Cookie AutoDelete, mostly optional but clears cookies while browser is running and not only on close
• DarkReader, so bright pages go dark
• Sidebery, for organizing tabs (I miss true tab groups in Firefox, but only a bit)

Firefox actually has most privacy stuff you need built-in nowadays. There are surprisingly few steps you need to harden it after install (on both desktop and mobile):

• Install the uBlock Origin extension.
• Switch Enhanced Tracking Protection to "strict".
• Turn on HTTPS-only mode in all tabs.

Optionally:

• Switch your search engine away from Google. I've been using DuckDuckGo with zero problems for years, but there are others.
• Install the multi-containers extension, it can be used to load websites in isolated color-coded tabs so no data "leakage" can occur.

You do not need any other extension. There is some advanced stuff for fingerprinting protection but they can do more harm than good if you don't know what you're doing. Stick to the above, update Firefox when prompted and that's all.