User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:47.0) Gecko/20100101 Firefox/47.0 Build ID: 20160604131506 Steps to reproduce: 1. Opened a page. 2. Went to Responsive Design View. 3. Tried to set width to 50px, by typing the width. Actual results: Nothing narrower than 280px was allowed. Expected results: It should have narrowed to 50px and potentially less. Using Responsive Design View, I've already been testing designs for viewports as narrow as 50px. Firefox recently supported this but now does not support less than 280px. I'd like the ability to set the RDV width down to 50px restored. It does not have to be a preset. I also wouldn't mind allowing narrower than that. One commenter in this context described a specialized use case for far less than 280px, and I can imagine both equipment with limited-purpose displays and Web pages inside frames (frames are deprecated in HTML but still common). I guess it was lost because it seemed useless until a new popular device requires it. However, I prefer to do my designing all at once and to see how my pages look under various constraints that may come down the pike later. It's more efficient than going back to websites later. And it avoids the problem of a user device coming out and becoming popular before Firefox catches up to the new spec. I do not have a tablet or a phone with FF, so I don't know how that works. Right now, to do my testing in Firefox down to 50px, I have to use a 2016 OS without updates, risking security. That's not good.

Thanks for filing! At the moment, the limit of 280px is in place because: a. We assumed there wasn't much need for a size below that b. The current UX with a toolbar inside the viewport breaks at small widths I'd be happy to lift the width restriction, but first we'll need to adjust the tool's UX to allow for it.

Sounds good. Thanks.