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Is there any way in Firefox to show the dimensions of the current viewport? Ideally in the status bar and updating "live" as you resize the window, as this would be extremely useful for responsive layout testing!
I've used "MeasureIt" but that requires you to draw with a ruler, which is tedious and quite possibly inaccurate depending on the ability and mouse resolution of the user.
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You can reset the window size for Firefox back to its default settings by deleting the xulstore. json file. It can be found in the root of your Firefox profile folder. You will need to make sure that Firefox is not running when you remove this file.
You'll notice dotted lines appearing around elements on your page. Find the content area and click to focus on it. Back in the bottom window, click Box Model on the right. The width and height will be shown.
resistFingerprinting enabled is intentionally resizing your window at start so that websites can't tell the exact monitor size... after disabling that, it seems the window size remains on boot.
I know this is a 4 year old answer, but I don't see anyone mention that you just have to select the toggle rulers option. Go to the settings menu option in the developer tools and check it. That will bring up the ruler icon and just click it and you have your ruler. 
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The popular Web Developer Extension has a "Display Window Size In Title" function (under the "Resize" menu) which updates when resizing. This displays both the window size and the viewport size in the following format: 1024x768 [1008x529] (window size, viewport size)
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