Here be tweaks I usually have for Firefox:

• apz.overscroll.enabled: false
• browser.compactmode.show: true
• browser.eme.ui.enabled: false
• browser.newtabpage.enabled: false (disable that stupid grid of tabs)
• browser.search.suggest.enabled: false
• browser.sessionstore.restore_on_demand: false
• browser.sessionstore.restore_hidden_tabs: true
• browser.sessionstore.restore_tabs_lazily: false
• browser.urlbar.trimURLs: false (returns the http:// in the URL bar)
• clipboard.plainTextOnly: true
• dom.allow_cut_copy: false
• dom.event.clipboardevents.enabled: false
• dom.image-lazy-loading.root-margin.*: 1000000 (faster browsing)
• dom.iframe_lazy_loading.enabled: false (same as above)
• extensions.pocket.enabled: false
• media.autoplay.enabled: false (yeah no) Not sure what the replacement is
• media.block-autoplay-until-in-foreground: true (well)
• media.eme.enabled: false (yeah no)
• media.gmp-widevinecdm: false (idem)
• media.gmp-widevinecdm-visible: false (idem)
• mousewheel.min_line_scroll_amount: 50 (faster scroll in a container)
• mousewheel.system_scroll_override_on_root_content.enabled: false (prevent overly fast scrolling because of previous setting)
• network.IDN_show_punycode: true (be safe)
• network.proxy.socks_remote_dns: true (ensuring dual-stack sites are accessible over)
• permissions.default.shortcuts: 2 (disable sites from capturing browser shortcuts)
• privacy.resistFingerprinting: true breaks Rikaichamp.
• toolkit.telemetry.unified: false
• toolkit.telemetry.*.enabled: false