Today I needed a specific really old #Firefox ESR version (don’t ask, answer may be unsettling) and look, stuff is all still on the Mozilla attic: https://ftp.mozilla.org/pub/firefox/releases/45.0esr/linux-x86_64. Neat! Somewhat dusty but it still starts flawless (on a tmp profile, of course) 🙋
Create a temporary folder for a temporary profile, Run Firefox with argument -profile path/to/tem...
Neat. Boils down to:
PROFILEDIR=`mktemp -p /tmp -d tmp-fx-profile.XXXXXX.d`
firefox -profile $PROFILEDIR -no-remote -new-instance
Doing something similiar with Chrome for Debug Sessions with vscode already.