The Flaming Bird Disassembler is the most powerful Apple IIgs disassembler in the galaxy: expandable, fast and reliable, it allowed me to disassemble the whole Apple IIgs System a long long time ago.
The Flaming Bird Disassembler is a product of the French group The Phoenix Corp and was written by Ferox in the 90's.
Version has been upgraded to 1.0.3. It contains a one-byte change that makes TFB Disassembler Sweet16 compatible. I am happy ;-)
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The manual in English (thanks, Dagen)
Link to the complete original manuals