AIUI, there are two ways to run Mozilla on Macintoshes:
The first is
CFM (Code Fragment Manager). This method was fully *carbon, with a Mac backend (Gecko, plus a few other bits) and a Mac frontend (what you see on screen). This build ran on both OS 9 and OS X.
The second is
Mach-O. As with CFM, it's carbon*, with a difference. Instead of a Mac backend, there's a UNIX backend, which I believe is common with the BSDs, Linux, etc. As with CFM, there's a Mac frontend. This build runs on OS X only.
Any Phoenix for OS 9 would have to been done via CFM. I believe that the Phoenix devs dropped CFM before Mozilla did as a whole. This isn't to say it's impossible, but I doubt it could happen.
*
Carbon is a way for a program to run on both Mac OS 9 & X. See
this for more.
UserAgent: Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-US; rv:1.7.2) Gecko/20040803