The interview said it was Ignor.
From the commentary of "Mother's Day" 2ACV14:
Matt Groening: So, this was an idea that we had before the series, that-
David X Cohen: That Mom and the Professor-
MG: -had had a romance.
DXC: Yeah, and we have some other ideas that may come up down the line linking into that...
DXC: We'll keep those deep in our hats, right now.
Billy West: See, this is sort of palatable, the idea of these two getting together, but then...
Rich Moore: It gets worse, folks.
He only said about Ignor in the interview after it was unlikely that the series would continue - bearing in mind that commentaries were recorded about 9 - 10 months before European release.