Not for responsive design Rated 5 out of 5 stars
EDIT after discussion with the developer :
This tool is set up to measure the size of the entire screen (and does it perfectly), but not for the size of the viewport, as needed for responsive designe development. Please take this in account and use Mozilla developer tool for responsive development preferably.
Five stars for developer help and quick answers !
OLD review : Doesn't work perfectly, adds some extra width due to the scroll bar. Anyway Firefox now has a responsive design option in its developer tools...EDIT (reply to dev answer) : in W7 - Firefox V18.0.2, when you compare the developer view (CTRL-SHIFT-M) width value and the firesizer width value: you should get very different pixel width values... I can PM you more info or help to debug if you wish.
I would be glad to address whatever you think "doesn't work perfectly". I have not had any other reports of issues with this extension.