Measure-it oleh Trishul

Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.

Keyboard shortcuts
Mac - Command+Shift+O
Windows/Linux - Alt+Shift+M
This can be changed from Add-on preferences.
Features

• Fast
• Simple
• Customizable UI/shortcuts from preferences

Note - This may not work on addons.mozilla.org