I did a back of envelope calculation in December 2016, and was surprised to find Firefox was smaller than Chrome that I kept around for testing. Not a bit smaller, half the size.

These were the original results:

$ du -sh /Applications/Firefox.app/
==> 189M    /Applications/Firefox.app/
$ du -sh /Applications/Google\ Chrome.app/
==> 366M    /Applications/Google Chrome.app/

I ran it again today:

$ du -sh /Applications/Firefox.app/
==> 179M    /Applications/Firefox.app/
$ du -sh /Applications/Google\ Chrome.app/    
==> du: /Applications/Google\ Chrome.app/: No such file or directory

Oh yeah, I got rid of the latter entirely. Still, Firefox is even smaller now.