Firesizer
Firesizer is a Firefox Extension.
You can download it from the Firefox Add-Ons site.
This extension allows you to instantly resize your browser window to specific dimensions.
After installing Firesizer, you’ll notice the current window dimensions in the status bar:
Everytime you resize your window the dimensions will adjust to reflect the changes.
When you right-click on the dimensions in the status bar, you’ll notice a set of default sizes that you can choose from:
You can add your current window size to the list by clicking “Save current size”:
Click on “Customize” to manually add/delete window sizes:
