Maximize/"Harden" Firefox privacy settings
(via Mental Outlaw YT channel and Chris Xiao's Yet Another Firefox Hardening
article and Unix Sheikh's Choose your Browser Carefully article):

Install Ublock Origin
Install Decentraleyes
Install LocalCDN
Install ForgetMeNot
Install NoScript
Install OneTab
Install Return YouTube Dislike
Install DuckDuckGo Lite Search
Install SponsorBlock for Youtube-Skip Sponsorships
Install ClearURLs
Install DarkReader (in DarkReader menu under More, set to Static)
Install Vimium-FF
Install Manjaro Theme (matcha-dark-azul for blue, matcha-dark-sea for green)
Set Default Search Engine to Duck Duck Go Lite
Get Firefox to remember nothing through Preferences setting
Set Firefox to never visit non  https websites
Disable smooth scrolling

about:config settings
(Optional): Turn off Javascript
media.peerconnection.enabled set to false #breaks real-time audio/video
media.navigator.enabled to false #breaks real-time audio/video
media.gmp
devtools.onboarding.telemetry.logged to false
datareporting.policy.dataSubmissionEnabled set to false
app.normandy.enabled set to false
extensions.pocket.enabled to false
extensions.formautofill.available set to blank
extensions.screenshots.disabled set to true
extensions.webcompat-reporter.enabled set to false
privacy.resistfingerprinting set to true #noticeable performance and stability impact, proceed with caution.
privacy.firstparty.isolate to true
network.dns.disablePrefetch to true
network.predictor.enabled set to false 
network.prefetch-next to false
network.http.sendRefererHeader to 0 #sites with forms and logins may break
network.http.referer.XoriginPolicy to 1
network.cookie.lifetimePolicy to 2
pdfjs.enableScripting to false
identity.fxaccounts.enabled to false
geo.enabled to false
dom.webnotifications.enabled to false
security.ssl3.rsa_des_ede3_sha set to false
security.ssl.require_safe_negotiation set to true
security.tls.version set to at least 3 (4 highest setting as of 2021, but not necessary according to Mental Outlaw)
security.tls.enable_Ortt_data set to false
browser.shell.checkDefaultBrowser = false
browser.formfill.enable set to false
browser.cache.disk.enable set to false
browser.cache.disk_cache_ssl set to false
browser.cache.memory.enable set to false
browser.cache.offline.enable set to false
browser.cache.insecure.enable set to false
browser.privatebrowsing.autostart set to true
browser.urlbar.speculativeConnect.enabled set to false
browser.search.suggest.enabled set to false
plugin.scan.plid.all set to false (same, couldn't find)
browser.ping-centre.telemetry set to false
browser.newtabpage.activity-stream.feeds.telemetry set to false
browser.newtabpage.activity-stream.telemetry set to false
browser.newtabpage.activity-stream.section.highlights.includePocket to false
browser.tabs.crashReporting.sendReport to false
browser.uidensity to 1 (makes the browser's tabs not so huge for firefox 89+)
devtools.onboarding.telemetry.logged to false
toolkit.telemetry.enabled to false
toolkit.telemetry.server Delete the URL and leave it empty
toolkit.telemetry.archive.enabled set to false
toolkit.telemetry.bhrPing.enabled set to false
toolkit.telemetry.firstShutdownPing.enabled set to false
toolkit.telemetry.newProfilePing.enabled set to false
toolkit.telemetry.unified set to false
toolkit.telemetry.updatePing.enabled set to false
toolkit.telemetry.shutdownPingSender.enabled set to false
webgl.disabled set to true
privacy.firstparty.isolate set to true
security.ssl.enable_false_start set to false

go into ublock origin and check the 'Prevent WebRTC from leaking local IP addresses' box.
go into usr/lib/firefox/distribution and open/create package.json and input the following:

{
  "policies": {
    "DisableAppUpdate": true,
    "DisableFirefoxAccounts": true,
    "DisableTelemetry": true,
    "DNSOverHTTPS": {
      "Enabled": false,
      "Locked": true
    },
    "DontCheckDefaultBrowser": true,
    "NetworkPrediction": false,
    "PromptForDownloadLocation": true,
    "SearchEngines": {
      "PreventInstalls": true
    },
    "SearchSuggestEnabled": false,
    "NetworkPrediction": false
  }
}

The following isn't privacy related, but turns off proton in firefox, which makes better use of screenspace:

browser.proton.enabled set to false
browser.proton.modals.enabled set to false
browser.proton.doorhangers.enabled set to false
browser.proton.contextmenus.enabled set to false