I would just tell you about a use case. Recently I wanted to debug an mxml file, so I downloaded the Adobe Flash Builder.And then built my mxml file there to swf format. Now I put a breakpoint in my mxml file at the relevant point and started my web app in Firefox, The application launched beautifully and I was able to see all the debugging data (variable values, call stack, etc) in Flash Builder. I was still analyzing those values and the program suddenly terminated. I looked at the Firefox window and it was showing some flash error. I tried to figure out repeatedly what was going wrong, but every time just after 15 seconds the program crashed and terminated.
Frustrated, I moved to IE for launching the webapp, And... problem solved... Now the program stopped crashing and I was successfully able to debug the app. What I learnt is, Firefox gives very less amount of time for Flash app to load (15-20 secs ), and we as human beings need more time to debug. SO point learnt, while debugging Flash applications, move to IE.