Measure-it v2.2.0 
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