when I did apt-get update / upgrade, there was firefox in to-be-updated packages. and what i did then was apt-get upgrade and type yes with firefox browser window open.
that's why my firefox on debian collapsed. or is that package originally broken?
I tried firefox reinstall and purge and remove again & again. So my conclusion is that... firefox deb package in PowerPC64 must be broken.
at first, I doubted it may be some error or break within firefox-gnome-support, but I failed also at fail-safe session. So, I'm disgusted.