What dates are you considering?
You need to be aware of QR Forum Rule No. 1: don't book to fly on QR for a specific type of aircraft as QR are a five stair airline for equipment swaps. Some routes are more "stable" than others, but I don't believe FRA is one of those.
There are raft of changes that happen in late winter and late summer when QR (and most other airlines) put the finishing touches on their summer and winter schedules. Whilst is is usually focussed upon timing changes, equipment is also thrown into the mix.
Looking at the seatmap on qrcargo.com from about a month before the flight is usually a good indicator with an increasing degree of certainity, but to a certain extent you can't be totally certain until you arrive at the gate.