Web Developer is actually a Firefox extension and literally has tons of key functionality to assist in website development. Given the UMPC motif lately, I’ll suggest it for folks that want to get a quick and dirty feel for web browsing in 800 x 480 resolution; the native res for the first generation of UMPC devices.
Just add the Web Developer extension and once installed to your Firefox client, click the Options button. Find "Resize" in the Options and add a new screen res; in my example, I called it "Native UMPC" and set it to 800 x 480. Click OK to save your custom browser size and then select the Resize button on the Web Developer toolbar. Choosing "Native UMPC" will resize your Firefox browser to 800 x 480, so you can experience web surfing on a UMPC-sized window. Keep in mind that UMPCs can also render at higher screen resolutions, but the 800 x 480 size is native.