Q: What is the ECL/CR of the First?

I’d provisionally put them at the following points:

Level adjustment for the First is +5 (though there shouldn’t be any PC First, generally speaking).

CR for the First is class level + 1 for characters of levels 1-5, class level + 2 from level 6 upward.

I haven’t ever used the First as combat opponents, so these are only ‘best estimates’. Use them with discretion.

Also, you may need to give a higher LA or CR for Firsts with Arcanist levels, as they get a huge spellcasting bonus.