I must agree with Beadsnfiber. I've just done about 8 HB sets in the last two months for a special order, and I use Nymo thread (single, not double), and I use delicas all the time. I use everything from round ones, to cylinder to hex beads. My first necklaces were very stiff, and I realised that my tension was really tight. So I loosened my tesion right down, just so the beads weren't floppy, and held together nicely. They're flexible and very wearable. I use them for bracelets and even bangles. Personally, I'd not use fireline - thin single nymo for me is the best.
Hope that helps.