Better Twitter - Hide 'Trending'.
Behind the overlay - Click to close any overlay popup on any website.
Clear URLs - Remove tracking elements from URLs.
Facebook Container - Restrict Facebook tracking.
Google Analytics Opt-out
Search by Image - configurable reverse image search.
View Image - Re-implements the google image, "View Image" and "Search by Image" buttons.
I don't care about cookies - Use this in conjunction with [x] Delete Cookies and Site data when Firefox is closed, in Firefox Settings, then you can clear out all the cookies by just closing the browser. [Website]
Firefox UI fix 1 + 2 Tweaks to remove the floating tabs. These involve creating a 'chrome' folder inside your Firefox profile and then placing a .css file with the customisations. Example userChrome.css with classic non-floating tabs.
Most of the default settings in Firefox are sensible, but there area few you may want to tweak, enter the url about:config on the address bar and then search for the setting to view or change the value.
browser.tabs.maxOpenBeforeWarn - Warning when opening more than N tabs at one time. Default is 15.
browser.tabs.warnOnOpen - Has to be TRUE for the above to have any effect.
browser.blink_allowed - Disable blinking HTML text.
browser.urlbar.trimURLs - Hide the protocol http in the url, Set to FALSE to always display the full url prefix.
browser.urlbar.suggest.topsites - Set to FALSE, equivalent to Settings > Privacy&Security > Address Bar > Shortcuts.
browser.newtabpage.activity-stream.default.sites - Edit the top social sites pinned to the Awesome bar.
browser.newtabpage.activity-stream.feeds.topsites - Set to FALSE to disable the top sites completely.
dom.max_script_run_time - Timeout for slow scripts = 5 (seconds) default is 10.
dom.event.clipboardevents - Set to FALSE to prevent websites from disabling copy/paste or reading/replacing the clipboard.
extensions.pocket.enabled - Set to FALSE to remove the context menu options Save to Pocket...
network.IDN_show_punycode Set to TRUE to display deceptive unicode URLs as punycode like Google Chrome does.
privacy.trackingprotection.enabled - Mozilla Tracking Protection
Link prefetching will by default pre-load all linked pages on pages that you visit. This can allow outside parties to profile your browsing habits from your DNS traffic, or allow an ISP to infer the web pages you visit within secure sites by looking at the prefetch resources.
Turning this off may lower performance:
network.dns.disablePrefetch = true
network.predictor.enabled = false
network.prefetch-next = false
Disable prefetch link on hover:
network.http.speculative-parallel-limit = 0
To get a warning before opening any non-secure pages:
Settings > Privacy & Security > Scroll to Bottom > Enable HTTPS-Only Mode
Some websites will use service workers to cache content. Many of these will just disappear when you close the tab for that website.
In a few cases you may need to scroll down the list below and unregister.
Add an exception on the Servers tab in the Certificate Manager. Options/Preferences -> Privacy & Security -> Certificates: View Certificates
How to restore the old Firefox address bar - requires a UserChrome.css entry.
List of Firefox chrome URLs and the related menu option.