Firefox Responsive Design feature

I upgraded to Firefox 31 today and I noticed a new feature (it may have been there a while? but it's the first I noticed it).

So, enjoying making my latest example of AngularJS and Worklight I was doing a Preview when I forgot that I'd switched my default browser to Firefox.

 I was thinking, why would this be useful? I can just resize the browser, but, if like everyone else I have at least 5 tabs open at once and well....it resizes all those two.  So, this kind of has it uses for a quick check that the style.css file you've been working on is working as expected for each size :-)

and if you want to cater for a CUSTOM size then you can move these about and make it fit.

Obviously, if using Worklight Studio...I could just do a Preview of the app using the Browser Simulator...

...and you can have multiple on the screen at once.