There is a download for Firefox for those users in the UK (rather than the default US English Version). It can be downloaded from: http://download.mozilla.org/?product=firefox&os=win&lang=en-GB. Found via: http://www.mozilla.org/products/firefox/all.html. You can also set your homepage to Google UK, with Firefox branding: http://www.google.co.uk/firefox?client=firefox-a&rls=org.mozilla:en-GB:official
As well as the official extensions site (https://update.mozilla.org/extensions/?application=firefox), there is also a site with many more extensions listed and available for download: http://www.extensionsmirror.nl/index.php?showforum=2
You will have to add www.extensionsmirror.nl to the list of sites that are allowed to install software. To do so, find an extension you want (i.e. ConQuery), click Install then click Edit Options in the notification bar and then click Allow, OK. Click Install again and the extension will install. You will not have to do this for any additional extensions on the site. As always, exit (via File - Exit, not the cross in the top right) then reopen the browser to use the extension.