Simulate Offline Mode for HTML5 Cache Testing

In Chrome you can open developer tools switch to the network tab and set throttling to offline

For Firefox
☰ (hamburger menu)->More->Work Offline

Google Chrome doesn't appear to have this feature

Edit: Also, another alternative is slightly more time-consuming to setup in the beginning but might be worth it. For Firefox/Chrome there should be proxy plugins, set up a fake/bad profile for each so that you proxy to something that doesn't exist... like 127.0.0.20:8080. After that you can switch the proxy setting on and off to emulate a full-stack test.

For Firefox, from the ☰ (hamburger menu), choose Web Developer > Work Offline.

For Chrome, open DevTools and select the Network panel. Throttling is set to Online (Disabled) by default. From the dropdown menu, you can choose presets Fast 3G and Slow 3G, but to simulate Offline Mode, you want to choose Offline.

The other answers are out of date. The only place this appears in Firefox v92 is under File > Work Offline.

The File menu can be accessed in the top left of Firefox by pressing the ALT key.