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.

Tip: How to run a new Firefox instance w/ temporary profile (cat_in_136.blog())

Create a temporary folder for a temporary profile, Run Firefox with argument -profile path/to/tem...