Video DownloadHelper - Download video files.
Google Search-Link fix - Don't rewrite urls in search results
Flash Block - Block Flash content, click to download and view only when required, or whitelist sites you visit regularly.
Enable pipelining (if you have a fast connection)
Open a new tab in Firefox (Ctrl+T) and type about:config in the address bar.
In the “Filter:” search box, type network.http.keep-alive and make sure it’s set to “True”
In the “Filter:” search box, type network.http.pipelining and make sure it’s set to “True”
In the “Filter:” search box, type network.http.proxy.pipelining and make sure it’s set to “True”
In the “Filter:” search box, type network.http.pipelining.maxrequests and make sure it’s set to 8 (valid values are from 1 to 8)
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 may need to 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.