Need to browse a site at a particular screen dimension? Launch Firefox, then press
Command + Option + M (on a Mac) to enter Fixed Screen Width Mode.
Different dimensions (e.g. 320 x 480) are available via the presets dropdown or drag the sides or corners to customize the dimensions.
Click the arrow button to change orientation or the camera button to take a screen capture. Click the X to return to variable-width browsing.
It’s not the same as browsing a site on an actual device but it’s very handy for an initial review.