Today I found a nice trick that I wish I would of realized years ago. I figured I'd post it here in case anyone else is missing out on this great Firefox feature for mobile developers and responsive web design. Firefox as a mobile screen display which you can get by using the shortcut:
CTRL + SHIFT + M
It's that easy.
To use this feature of Firefox, simply open the browser, hit the key combo, control+shift+m and type in any web address in the address bar like normal. Here's what it looks like:
...and what's really nice is that you can use developer tools while at this size and be able to read them easily with little scrolling. I used to try to use the tools while the browser window was very small and I couldn't read more than a line at a time but now it is easy. Here's a picture of the same website as above with developer tools opened: