Fire Resizer от Francesco De Stefano

Add-on to resize the browser window.

With quick presets for mobile devices, check media queries or your mobile website with a single keystroke.
You can set your own custom presets for your devices.
Your presets will be synced across all devices currently logged into your account.

Keyboard Shortcuts
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The extension allows for numerous keyboard shortcuts to speed up development:
* Ctrl + Right Arrow - opens the extension popup
* Up - manually increases browser width
* Down - manually decreases browser width
* Ctrl + 1-0 - resizes to saved preset
* Ctrl + Shift + 1-0 - resizes to the landscape/inverse of saved preset
* Ctrl + Esc - resets browser window to 1200x800

Cmd is substituted for Ctrl on Mac.

NOTE: Because Cmd + Shift + 3 and Cmd + Shift + 4 on a Mac take a screenshot, landscape orientation for the devices in position 3 and 4 won't work on a Mac.