FlashTracer - Firefox Extension which helps you debug.

When you deploy your application online you actually unable to debug it properly, hard to see messages and any other outputs, except Alerts. Luckily I've found Firefox extension which can help me a lot. FlashTracer is project of Alessandro Crugnola, http://www.sephiroth.it. You can download it from Firefox Addons website or directly from its home website - http://www.sephiroth.it/firefox/flashtracer/.
FlashTracer is very intuitive and easy to set up.
But fist of all you need to install Flash debug player in order to have it working - http://www.adobe.com/support/flashplayer/downloads.html

Setup is very simple you need to choose place of log file, then change some setting related to Flash Logging in configuration file:

Very powerful feature is filtering of trace output and ability to use REGexp with CSS to make certain thins of certain color.

Keep in mind this plugin displays you whole Flash virtual machine trace output, not only Firefox's. So, for example: application which running under Opera will have all its output displayed in you FlashTrace window. Which is pretty cool because Opera's extensions really suck. But it will not gonna work with application launched under IE, because IE uses Active X component, whic will be running in separate from Firefox process.