All web browsers relay some information to the web servers when you visit a website including the name of the web browser, its version, and the platform on which you are using it. This information is sent in form of user-agent strings and can be used to identify your web browser and the operating system. Some websites can use this information to deliver you platform or browser specific webpages. But if you want to modify this user-agent string, then it is possible to spoof it and fool the web servers into believing that you are using a different web browser from a different platform.
In the Firefox web browser you can spoof the user-agent strings with the help of User-Agent Switcher & Manager extension. This extension allows you to select from dozens of the user-agent strings available for a number of web browser and platforms. After the installation of the User-Agent Switcher & Manager extension you can click on the small icon in the toolbar and choose one of the user-agent strings available. You can choose a web browser, a platform and even the browser versions from the list before clicking on the Apply button.
While some of the other similar extensions apply the spoofed user-agent strings globally to all the tabs, this extension can be set to work globally or for only for particular domain names. In the settings for the extension, you can also set up white lists and black lists. In the black list mode it sets the spoofed user-agent strings for all the domains except the ones in the list. In the white list mode, it sets the spoofed user-agent strings for only the sites in the list. You can change these modes by right-clicking on the User-Agent Switcher & Manager extension icon in the toolbar.
You can get the User-Agent Switcher & Manager extension for Firefox from https://addons.mozilla.org/en-US/firefox/addon/user-agent-string-switcher/.