Unsecured Updating (these extensions) - Starting in Firefox 3, if an add-on does not provide a secure method of auto-updating then, by default, Firefox will refuse to install the add-on. To use these extensions you will need to add the preference extensions.checkUpdateSecurity and set it to false in about:config.
Toggle Proxy (v1.1) - designed to toggle between two proxy settings without having to go through the preferences/options dialogue. Adds an icon in the status bar. Left clicking on the icon will toggle between two different proxy settings set in the preferences. Right clicking will open the preferences window where you can select which proxies to toggle between.
Update: 3/12/2009 - Now available via Add-ons for Firefox on the Mozilla website, search for Toggle Proxy.
Update: 27/5/2011 Toggle Proxy (v1.2) released. Added a shortcut key (alt + t) to toggle between the two settings and a French locale. Working on creating a user specified shortcut key but it's not as easy as I thought so could take a few weeks to do! Also available on the Mozilla add-ons site and Softpedia who have tested it and have found it to be: "100% FREE award granted by Softpedia"
Update: 29/5/2011 Toggle Proxy (v1.3) released. Changed shortcut key to (alt + x) to toggle between the two settings. Didn't realise that alt + t would cause a conflict on Windows, I develop on OS-X! Now tested on Windows, works on there as well as OS-X, still plan to let the user select the shortcut key.
Update: 29/7/2011 Toggle Proxy (v1.4) released. Fixed a problem with the French locale and added a Dutch locale to the addon.
Update: 13/1/2012 Toggle Proxy (v1.5) released. Added support to Thunderbird.
Update: 30/11/2012 Toggle Proxy (v1.8) released. After much prevarication added a toolbar icon for Firefox - wasn't as hard as I thought it was going to be!