Browser plugin to resize window for brower testing

Solution 1:

Was just digging around in firefox and found that it also has a "Responsive design" mode. Go to Web Developer → Responsive Design View.

You'll get a dropdown with preset resolutions afterwards:

Solution 2:

This is actually a built-in feature in Chrome.

1. Open the DevTools (F12)

2. Open the Settings

   

3. Set the size you want for your window in the Overrides panel.

   

Note: When selecting a User Agent in that panel, Chrome will automatically try to pick the correct device metrics for the selected agent.

Solution 3:

For Chrome you can use Window Resizer. Fore Firefox, you can use Firesizer.