Firefox does offer a good amount of privacy settings in preferences, but not everything can be adjusted with those settings.
With this "guide" you'll be able to set up your Firefox as mine: Cookies won't be saved and any unnecessary cookies will be blocked. Also measures are taken so the browser won't leak your IP address (even with the usage of VPN).
It does not only work with Firefox alone, but also browser based on Firefox (e.g. Waterfox).
First you have to enter about:config into your address bar. Accept the warning and now you are able to access all the little settings.
Change the values off the settings like seen below:
browser.cache.offline.enable = false
browser.send_pings = false
dom.battery.enabled = false
dom.event.clipboardevents.enabled = false
geo.enabled = false
media.navigator.enabled = false
media.peerconnection.enabled = false
network.cookie.cookieBehavior = 1
network.cookie.lifetimePolicy = 2
network.http.referer.trimmingPolicy = 2
network.http.referer.XOriginPolicy = 2
network.http.referer.XOriginTrimmingPolicy = 2
network.prefetch-next = false
privacy.firstparty.isolate = true
privacy.resistFingerprinting = true
privacy.trackingprotection.enabled = true
privacy.trackingprotection.fingerprinting.enabled = true
privacy.trackingprotection.cryptomining.enabled = true
webgl.disabled = true
Some functions of these AddOns are already covered by the settings adjusted above, but in my opinion it is better to have these running.
uBlock Origin: A wonderful little helper for content filtering that also wonderfully blocks ads.
HTTPS Everywhere: An extension that makes your browser use the more secure HTTPS connection, as long its available. It can also block non-https connections. Note: HTTPS Everywhere is not necessary anymore. Firefox has the option to enforce https and/or block http added to it by now
Decentraleyes: Blocks direct traffic to Content-Delivery-Systems which can be used for tracking. Don't worry, things will be working as before, because it'll use local libraries.