Handmade Particicle System

I had to develop a particle system in flash which could render more than 20.000 particles.

Check the final solution. That is not perfect, but it is really near to the best you can run on AS3.

The low level implementation throught image processing can render up to 50.000 particles moving.

Automated Testing using Selenium

For browser-based testing of web applications I recommend Selenium. It can be used both for functional, compatibility (it has extensive cross-browser support) and regression testing.
I used the Java Api, but it is available in other languages.
There are other tools similar to Selenium.
In my team we use it and we feel quite comfortable but for a brand new project I would rather review some other tool. Anyway Selenium is great.