Firesizer is an extension for Firefox that will let you pre-set a series of browser sizes so you can quickly jump from one to the other. Here is a little more information about the extention:

Provides a menu and status bar to resize the window dimensions to a specific size. Unlike other similar extensions, this one sets the size of the *entire window*, not just the HTML area.

Nobody is going to claim this extension is reinventing the wheel, but it is a small extension that does what it sets out to do. As long as you can get that much done, your alright in my book.