Probably the optimum testing method for a mobile site is:
* Test the site in a desktop browser first to ensure functionality behaves as expected
* Once the site is functionally complete, test using some device emulators
* When the site is working correctly across all emulators, begin testing on real handsets
See more in:
Testing Mobile Web Sites Using Firefox
More information about useful Firebox Web Mobile related Plugins:
Firefox Plugins for Mobile Web Developers
XHTML (MP) Mime Mobile Profile
Firesizer One of many resizers
Mobile Phone Identification Information Sources
Mobile Browser ID (User-Agent) Strings
User Agent Profile (UAProf) Lists Here, Here and Here
