Firefox

Websites automatically adapting to viewport size is pretty handy, not having to select whether you want the mobile site or not each and every time you load the page is generally considered a good thing.

You may also want the website to adapt to smaller or wider windows, unless you want every website to become one where you manually zoom in and pan around.

Similar things go for language and timezone.

There are various ways to spoof various settings about your browser, through add-ins or otherwise.