William Morder composed on 2018-03-24 08:18 (UTC-0700):
A friend told me that he creates separate partitions for both /opt and /etc; and then, too, I noticed that the new Icecat browser does something even weirder, which is to install in /usr/lib/icecat, rather than (like other Mozilla browsers) in /home/<USER>/.mozilla/.
Mozilla browser *profiles* (user data) traditionally go in ~/.mozilla (/home/username/.mozilla). On my systems this is Firefox and SeaMonkey. Thunderbird's go in ~/.thunderbird.
Mozilla predates other common GUI web browsers by a bunch of years, when users' appdata went in $HOME by convention. Newer browsers, eg $HOME/.config/chromium, have adopted a superceding convention, $HOME/.config/appname/.
/usr/lib/appname is where most apps go, main exception being those that inexplicably decide on /opt/ instead.