I am very curious about Firefox's "responsive design viewer" (which I was very slow to discover as it appears to have around for some time.) I would ask this in the developers forum but I don't know my way around "over there." First I LOVE RDV! I discovered it just when I needed it in my work and making several websites "mobile friendly." I have used it now extensively and realize that it is far different then simply resizing browser dimensions as it responds to all the CSS3 media queries, whereas simply resizing any browser (even Firefox) will only respond to some of them and makes all those bogus "put your webpage in our frames" emulators to shame. It is MORE than what is said on the developer page: https://developer.mozilla.org/en-US/docs/Tools/Responsive_Design_View "Of course, you could just resize the browser window: but making the browser window smaller makes all your other tabs smaller too, and can make the browser's interface much harder to use." You can see an example here, if you don't mind a link (and if you do just remove it): http://www.wcwcw.com/formmailer/form.php O.K., two things are happening that don't happen when you just resize a browser or use bogus frames emulators. First, the menu, that takes up most of the initial screen, has its font enlarged (to try and satisfy Google PageSpeed Insights separation of links...still working that out) and 2nd, the text input box is resized by CSS3 media queries so it fits into the viewport. Neither of those things happen in "resizing" the browser but do render in RDV. So I am extremely interested in how this is being accomplished. Something more than a convenient way to resize browser windows is going on. I am not asking for trade secrets or anything. I am just interested in how RDV in Firefox can successfully do that! RDV is the only tool I have that "agrees" with Google PageSpeed Insights about view ports. I am just crazy curious to understand what is going on ;-) (and I hope that doesn't turn into an "emoticon.") Kind Regards, Axis