To install the plug-in, search for “User Agent Switcher” under “Tools->Add-ons” in Firefox.
Of course, this is the cart before the horse since you would need to know the UA of the handsets before populating this tool. There are several means to do this: (a) use the handset browser to hit sites such as http://whatsmyuseragent.com/ and http://www.useragentstring.com/ (b) write a simple PHP script to log down (or capture) the HTTP headers – and just look for the header “User-Agent”.
If you’re just starting up and need to start a collection of UAs, there are several sites to start with: (a) http://zytrax.com/tech/web/mobile_ids.html (b) http://en.wikipedia.org/wiki/List_of_user_agents_for_mobile_phones (c) http://www.useragents.org/ (d) http://en.wikipedia.org/wiki/Wurfl or www.wurfl.org. WURFL is probably one of the most useful resource for device information including UA strings.
Most of the time, we have to test for page rendering and functional requirements. To perform both, the ideal would be to use an actual handset. However, this can be extremely time consuming (even HSDPA speeds do not match WiFi speeds and handset processors can also be slow) although it gives a good feel for the actual user experience; also, there’s always a shortage of test handsets to go around – hence it makes sense to simulate the device browser. This will be covered in the subsequent posts.