modern computers start with 8gig of memory
Is the ASUS Transformer Book, a 10" convertible laptop computer, not "modern" because it ships with only 2 GB of RAM?
and then there's the page file/swap space.
A lot of computers without a rotating hard drive, such as my Nexus 7, can't afford to swap. Instead, they have a harsher OOM killer.
Firefox is a browser meant for browsing... and if that's what you're doing with it, that 1gig of memory is nothing. What background apps are using up the other 7gig?
I'm guessing that when no application is using part of the RAM, the chipset could power down unused RAM to prolong battery life.