Works, but has a few wrinkles Rated 3 out of 5 stars
This is the only way—that I'm aware anyhow—to export RSS feeds from IE and import them into Firefox. (IE7 has its own OPML Import/Export functions.) The first time I tried importing into Firefox, the OPML Support Import dialog seemed to hang (dialog just stayed put). I only had two feeds, so it wasn't like it was overburdened.
I closed all browsers then re-exported IE7 RSS feeds to OPML (this time with more feeds) and then re-imported OPML into Firefox. The second time things went okay.
So there might be some interplay issues, but I'm not aware of a better option at this point.