It's easy, well it gets easier with repetition.
Go to hypem.com, find the song you like and play it to the end. Next open up a new tab and type in "about:cache". Scroll down to "disk cache device" and click "list cache entries". Ctl + F to find a file with .mp3 and the end. Click that file. Look at the file path under "file on disk", mine was file on disk:
/Users/ME/Library/Caches/Firefox/Profiles/***********/Cache/9E2FF640d01 find that file on your computer, copy it to desktop, and finally rename it to something with .mp3 on the end. DONE