Further reading:

http://kb.mozillazine.org/About:config_entries (Outdated)
http://thesimplecomputer.info/tscs-firefox-tweak-guide
https://www.privacytools.io/#about_config

----------------------------------------------------

SPEED

(Default values from Tor Browser 4.5.3:)

network.http.pipelining – true
network.http.pipelining.abtest – false
network.http.pipelining.aggressive – true
network.http.pipelining.max-optimistic-requests – 3
network.http.pipelining.maxrequests – 12
network.http.pipelining.maxsize – 300000
network.http.pipelining.read-timeout – 60000
network.http.pipelining.reschedule-on-timeout – true
network.http.pipelining.reschedule-timeout – 15000
network.http.pipelining.ssl – true
network.http.proxy.pipelining - true

network.http.max-connections - 256
network.http.max-persistent-connections-per-proxy - 256
network.http.max-persistent-connections-per-server - 6

network.http.redirection-limit - 20
network.http.fast-fallback-to-IPv4 - true
network.dns.disablePrefetch - true
network.prefetch-next - true

(End of Tor default values)

Enable new cache:
https://bugzilla.mozilla.org/show_bug.cgi?id=913807
browser.cache.use_new_backend - 1

----------------------------------------------------

SECURITY/PRIVACY

Disable WebRTC (extremely important for VPN users - WebRTC *will* leak your real IP address):
media.peerconnection.enabled - false
media.peerconnection.use_document_iceservers - false

Disable DNS proxy bypass:
http://kb.mozillazine.org/Network.proxy.socks_remote_dns
network.proxy.socks_remote_dns - true

Disable IPv6:
http://kb.mozillazine.org/Network.dns.disableIPv6
network.dns.disableIPv6 - true

Disable crash reporting:
http://kb.mozillazine.org/Breakpad.reportURL
breakpad.reportURL - (blank)

Disable sending pings:
http://kb.mozillazine.org/Browser.send_pings
http://kb.mozillazine.org/Browser.send_pings.require_same_host
browser.send_pings - false
browser.send_pings.require_same_host - true

Enable tracking protection:
privacy.donottrackheader.enabled - true
privacy.donottrackheader.value - 1
privacy.trackingprotection.enabled - true

Disable geolocation:
geo.enabled - false
geo.wifi.uri - (blank)

Disable geotargeting:
browser.search.geoSpecificDefaults - false
browser.search.geoSpecificDefaults.url - (blank)
browser.search.geoip.url - (blank)

Disable telemetry:
toolkit.telemetry.enabled - false
toolkit.telemetry.server - (blank)

Disable 'safe browsing' aka. Google tracking/logging:
browser.safebrowsing.downloads.enabled - false
browser.safebrowsing.downloads.remote.enabled - false
browser.safebrowsing.enabled - false
browser.safebrowsing.maleware.enabled - false

Type 'google' in about:config and delete most of the links (the ones that won't break anything, that's up to you).
Also search through these to delete links from:
browser.contentHandlers
browser.safebrowsing
browser.search
gecko.handlerService

Disable WebGL:
https://security.stackexchange.com/questions/13799/is-webgl-a-security-concern
webgl.disabled - true

Install unsigned addons in Aurora/Dev-Edition/etc (needed for Privacy Badger, HTTPS Everywhere, etc.):
xpinstall.signatures.required - false

----------------------------------------------------

APPEARANCE

Show full URLs:
browser.urlbar.trimURLs - false

Revert to old search bar layout:
browser.search.showOneOffButtons - false

Remove "(site) is now fullscreen" nag message and make it faster:
full-screen-api.approval-required - false
browser.fullscreen.animate - false

De-crap new tab page:
browser.newtabpage.directory.ping - (blank)
browser.newtabpage.directory.source - (blank)
browser.newtabpage.enabled - false
browser.newtabpage.enhanced - false

Disable tab animation:
http://www.askvg.com/how-to-disable-animation-while-opening-new-tab-in-mozilla-firefox-4-0/
browser.tabs.animate – false

Speed up security delay when installing add-ons:
security.dialog_enable_delay - 400

Enable eyedropper in dev tools:
devtools.command-button-eyedropper.enabled - true

Dark theme for dev tools:
devtools.theme - dark

----------------------------------------------------

BLOATWARE

Disable 'Reader Mode':
reader.parse-on-load.enabled - false
readinglist.server - (blank)

Disable 'Pocket':
browser.pocket.api - (blank)
browser.pocket.enabled - false
browser.pocket.site - (blank)

Disable 'Firefox Hello':
https://www.mozilla.org/en-US/privacy/firefox-hello/
loop.enabled - false

Disable 'Social' crap:
social.directories - (blank)
social.remote-install.enabled - false
social.shareDirectory - (blank)
social.toast-notifications.enabled - false
social.whitelist - (blank)

Disable PDF reader:
pdfjs.disabled - true
