This cost me precious time of my life, no one ever will give me back. Very short: some early versions of FF3.5 (and users don’t always update) do have following bug: dynamic textfields (with font embedded) don’t accept @ as an value, but display (and sends to DB) »q« (on Win) instead (I read, that it has problems with some more special characters).
Solution: turn off
wmode = "transparent"
for the SWF or force update to FF3.5.3