I get same error too.
Looks like installing "FlashFirebug" is the reason of this error.
Here is the solutions:
1) Quit "FireFox" when using FlashBuilder.
If FireFox is available behind FlashBuilder working, it catches SWF publishing and try to connect it (that causes error).
2) Edit "mm.cfg" file.
Delete the line "PreloadSWF=...\firstname.lastname@example.org\chrome\content\flashfirebug\flashfirebug.swf" thing from mm.cfg.
Either way works for me.
I just realized that you can solve the problem with the solutions I posted before though,
you are still messed up when using "Profiler".
You just can't exec FireFox while you do "Profile" with FlashBuilder
(editting mm.cfg won't work this time. PrealodSWF is set to FlashFirebug thing).
I don't recommned to install "FlashFirebug extension" for the time being.
The option to disable firebug and quit firefox didn't work for me, but yes removing the line
fromt mm.cfg (C:\Users\UserName\mm.cfg) worked for me.
Thanks for the solution.