Recently Adobe released the final version of flash player 9 for Linux, fortunately thay also released the linux debug version, so the Flash Tracer extension can be installed now on linux too.

First of all Download the Linux debugger and standalone players and install it.
Then download flashtracer.xpi (updated version for linux only) and install (usually download and then drag on FF).

Restart Firefox and open the FlashTracer panel using Tools->Flash Tracer.
Browse to a flash website with some traces enabled and you should alreay see some text in the panel. Otherwise set in the flash tracer option the output file to this folder:

/home/{username}/.macromedia/Flash_Player/Logs/flashlog.txt

* replace {username} with your account name.
Now I can see all the traces (and lot of warnings, more than on Windows) on my Firefox in Ubuntu.