For me, Ctrl+Shift+m loads the Responsive Design View tool, which lets you choose a variety of different screen resolutions.

Do you get a resolution chooser or just nothing happens?

If you use the menu, does the feature work or does it seem to be disabled? Either of these should call it up:

• "3-bar" menu button > Developer > Responsive Design View
• (menu bar) Tools menu > Web Developer > Responsive Design View

If the menu works but the keyboard shortcut doesn't, it's possible an extension is intercepting that shortcut, but tracking it down might take some work.

Are there any error messages or warnings showing in the Graphics section on the Help > Troubleshooting Information page?

When I use Ctrl+Shift+M combination it loads the Responsive Design View tool and I get resolution chooser for different screen resolutions. But, neither it loads responsive design nor I see any media query of CSS in firebug for my design.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

I noticed after changing the screen resolution that I need to reload the page in order for the scripts in the page to detect the change. If Firefox is caching the earlier state too tenaciously, you can try Ctrl+Shift+r to bypass cached files for the page.

This happens when I am working on local machine, mobile preview of my site shows properly when it is accessed from server. As per your saying Ctrl+Shift+R does not make any change to preview when working on local. If I resize browser window than it shows mobile preview of my local site.

Are accessing using http://localhost/ or file://? Some things do not work normally with file:// URLs.

Accessing my site with http://localhost/.