This add-on will let you change the user agent: https://addons.mozilla.org/mobile/addon/phony/

For Google Reader, you don't need to change the useragent; you can just go to http://www.google.com/reader/i/

Reader works well in Firefox, but many of Google's mobile sites use WebKit-specific markup, and will not work correctly in Firefox if you change the user-agent header to iPhone or Android.