Web development plugin for Mozilla and Firebird

I’m on holidays and in my role as diarist and photo taker I’ve needed to see what the resulting JAlbum pages look like on my parents and other, well, shall we say, “resolution challenged” monitors (compared to my 1400×1050).
A quick google search turned up the web developer extension which is a plugin for Mozilla and Firebird.
I’ve not had much of a chance to play with all the features, but it installed pretty easily, and appears to work pretty damn well. My initial need was to be able to change my browser to an 800×600 and 1024×768 with a minimum of fuss. It certainly does that, and lots, lots more.