Video Downloader Professional - Download video files, or for highest quality use youtube-dl
Google Search-Link fix - Don't rewrite urls in search results
Flash Block - Block Flash content, click to download / view.
Google UK - Add Google UK to the search bar.
Ad Blocking plugins can greatly increase the amount of memory used by Firefox.
If Firefox is running behing a firewall that blocks the mozilla update servers, then checking for updates can cause the browser to run very slowly until it times out. Checking for updates can be turned off under Preferences | Advanced | Update, but don't forget to check for updates manually if you do this.
Remove 'unremovable' Firefox plug-ins
Third party software can install add-ons and plugins to Firefox which you don't need or want. These do not use any system resources unless you happen to visit a page that triggers the plug-in, but they do present a small potential security risk.
Visit mozilla.org/plugincheck/ to see if all your plug-ins are up-to-date.
The full file path to each currently installed & enabled plugin is visible at about:plugins
The plugin files are generally DLLs, to disable a plug-in, delete (or rename) its DLL. The Firefox about:addons page will then display a [remove] button for the plugin. You can press F5 or restart the browser. Note in some cases this will affect ALL browsers not just Firefox.
While the above will remove most plug-ins, some such as Adobe Acrobat and Windows Media Player will attempt to re-add themselves when the browser is restarted. Turning off plugin scanning (http://kb.mozillazine.org/Plugin_scanning) can remove these plug-ins.
Alternatively in about:config search for plugin.scan.WindowsMediaPlayer and change the min version to 99
browser.tabs.maxOpenBeforeWarn - Warning when opening more than N tabs
browser.blink_allowed - Disable blinking HTML text.
dom.event.clipboardevents - Set to FALSE to prevent websites from disabling copy/paste.
privacy.trackingprotection.enabled - Mozilla Tracking Protection