Hey no prob - I've swept through a new install of nightly on another system to get a concise set of ideas together (personal preference / UX tweaks have a † prepended):

#GUI configs:

about:preferences#general

• † check Open previous windows and tabs
• † uncheck Ask before quitting with Ctrl+Q
• uncheck Use AI to suggest tabs and a name for tab groups
• † uncheck picture in picture video controls
• uncheck Recommend extensions as you browse
• uncheck Recommend features as you browse
• uncheck Enable link previews

about:preferences#home

• uncheck Recommended stories
• uncheck Sponsored shortcuts
• uncheck Sponsored stories (disabled when you uncheck Recommended stories)

about:preferences#search

• Default Search Engine: duckduckgo (or whatever you'd prefer)
• uncheck Suggestions from Nightly (Firefox)
• uncheck Suggestions from sponsors
• Search Shortcuts (remove google, bing, ebay, etc)

about:preferences#privacy

• uncheck Send technical and interaction data to Mozilla (I don't usually offer my information as my behaviour is a poor representation of end users)

• uncheck Allow personalized extension recommendations (disabled with the above)

• uncheck Install and run studies (disabled with the above)

• uncheck Send daily usage ping to Mozilla

• https://addons.mozilla.org/en-US/firefox/

• https://addons.mozilla.org/en-US/firefox/addon/ublock-origin/

• (install ublock origin and allow its use in provate browsing)

[right click and customise toolbar]:

• † remove view recent browsing across windows and devices
• † show bookmarks bar on new tab page only
• † customise gui to remove spacers, add home button

#about:config

• † browser.uidensity == 1 (thinner top bar)

falsify the following:

• browser.tabs.crashReporting.sendReport
• datareporting.healthreport.uploadEnabled
• datareporting.policy.dataSubmissionEnabled
• datareporting.usage.uploadEnabled
• security.csp.reporting.enabled
• toolkit.telemetry.reportingpolicy.firstRun
• browser.newtabpage.activity-stream.feeds.telemetry
• browser.newtabpage.activity-stream.telemetry
• browser.newtabpage.activity-stream.telemetry.ut.events
• browser.search.serpEventTelemetryCategorization.enabled
• browser.search.serpEventTelemetryCategorization.regionEnabled
• dom.security.unexpected_system_load_telemetry_enabled
• identity.fxaccounts.telemetry.clientAssociationPing.enabled
• network.trr.confirmation_telemetry_enabled
• nimbus.telemetry.targetingContextEnabled
• telemetry.fog.init_on_shutdown
• toolkit.telemetry.archive.enabled
• toolkit.telemetry.bhrPing.enabled
• toolkit.telemetry.reportingpolicy.firstRun
• toolkit.telemetry.shutdownPingSender.enabled
• toolkit.telemetry.unified
• toolkit.telemetry.updatePing.enabled
• browser.ml.chat.enabled
• browser.ml.chat.menu
• browser.ml.chat.page
• browser.ml.chat.page.footerBadge
• browser.ml.chat.page.menuBadge
• browser.ml.chat.shortcuts
• browser.ml.chat.shortcuts.custom
• browser.ml.chat.sidebar
• browser.ml.enable
• browser.ml.linkPreview.longPress