User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.119 Safari/537.36 Steps to reproduce: Opening devtools and activating Responsive Design Mode. Tried to reach 240*320px resolution since that's the one of the phone I'm working on. Actual results: Only reached 280*320px. This is because minimum viewport dimension is actually hardcoded inside Firefox source code. See https://hg.mozilla.org/mozilla-central/file/3ee38289dac8/devtools/client/responsive.html/constants.js Expected results: I would have expected to reach 240*320px. I can understand a minimum limit might be needed but maybe would it be better if it was a config property so that users can override it and go below 280px if needed.

Thanks for the report! The minimum is there because the UI would likely wrap / break strangely due to the device selector that's placed inside the viewport width. (You can surely argue that suggests the device selector should be moved, and we may indeed do that at some stage.) Out of curiosity, what device has this low width you're testing?

Yeah, I'm totally fine with hiding the device selector when going under 280px, don't really need it for that specific use-case. Moving it somewhere else would be better of course. I'm working on an app for the Jio phone (http://www.jio.com/en-in/book-jio-phone) > DISPLAY > Screen Size: 6.09cm (2.4inch) > Screen Type: TFT screen > Screen Resolution: QVGA (320 x 240) Thanks for explaining the reason behind it.

Some users require an even lower minimum size (e.g. 50px), see bug 1410665 for reference.

240px wide viewport are common on feature phones, which are still there, and even gaining marketshares in some countries like India: https://www.valuewalk.com/2018/05/feature-phone-ios-vs-android-report/ I would like to be able to resize the responsive view to as small as I want, and add custom devices with any viewport sizes.

The Apple Watch will now be able to show web pages, with a viewport 272 pixels wide on the smallest model: https://timkadlec.com/remembers/2018-06-06-web-watch/

We changed the minimum to 50x50 in Bug 1467572. Marking this as a dupe.