How to resize and lock Firefox browser windows for specific sizes?

Question

How would I re-size and lock Firefox browser windows for specific sizes?

For example, my resolution is higher but I want to fix the width to 768px and lock it.

Answer 1

I've used Sizer for Windows: http://www.brianapps.net/sizer/

From the web site:

Sizer is a freeware utility that allows you to resize any window to an exact, predefined size. This is extremely useful when designing web pages, as it allows you to see how the page will look when viewed at a smaller size. The utility is also handy when compiling screen-shots for documentation, using Sizer allows you to easily maintain the same window size across screen grabs.

Answer 2

There is Firesizer.