title_body: browser tools to test custom viewport sizes? I'm coding html for ebooks, and my first test is in the Firefox browser.  I need a down-and-dirty to simulate and compare web design in different device viewports in both landscape and portrait mode. 
  
  When  I use the web developer --> responsive design view in Firefox, I have some presets, and if I click the rotate button I can see both landscape and portrait mode. But I don't see any capability to add custom viewport dimensions... which is strange given all the devices which are coming out these days. 
  
  Is there a way to do this? Also, is there a good URL which lists all the most popular viewport sizes for devices?
upvoted_answer: Once you're in the responsive design view, the handles on the right side and bottom right corner of the view let you drag to set a display size. The drop-down menu in the upper left corner shows the dimensions as you drag. Once it's at the size you want, you can click on the menu, select Add Preset at the end of the list, and save those dimensions with a name.
  
  Here's a question that lists some resources for viewport dimensions: https://stackoverflow.com/questions/5784485/how-to-figure-out-common-viewport-dimensions-not-screen-sizes-of-common-device