Proto-Germanic reflex

Would the -u- have come from the syllabic r?

Yes. That's why the -i must have been attached later. If it had been there in the original form, *fri- would have resulted because the syllabic r would become consonantal next to a vowel. Compare *fra-.

Ah, okay. Now I'm remembering a PIE word *pr̥h₃. Is this correct? What part of speech is it reconstructed to be?

I don't know, but it would become *fur in Germanic.

Ok, thanks.