I take a bookmarklet over an extension any day, and this is one of those cases.

Yet Another Browser Resizer, is a simple Firefox extension that adds an item to the context menu with options to resize the window to one of four predefined window sized, which of course match some of the most popular screen resolutions around. Plus you can also add as many custom resolutions as wanted.
Nifty. But I found that you can do the same thing with some bookmarlets. Just bookmark the links below. When you need to resize the window, select the appropriate bookmarklet . Note that this won’t work if you have disabled the ability for scripts to resize windows (Tools/Options/Content/Enable Javascript/Advanced/Move or resize windows). I use to have that option enabled, so it works for me.

• Resize to 320 x 240
• Resize to 640 x 480
• Resize to 800 x 600
• Resize to 1024 x 768
• Resize to 1200 x 1024