how to add flash player on my website ?

Yeah, this is the wrong forum for discussions about content development. 

If the browser crashes when you add a gif to the page, you have bigger problems than Flash video.  I'd start by grabbing the latest version of Firefox and making sure your graphics drivers are current.  If your browser is still crashing and you're just adding .gfis, the Firefox support forums might be a better spot for help... either that, or just try a different browser like Chrome.

If you're using "crash" loosely, and you're instead just getting some kind of error message from JavaScript or PHP, you might start by just googling the section of the error that isn't specific to your application.

1) Flash is run on the client machine, not the server. no need to install anything on the server

2) Program using html 5 and use the video tag and produce your videos in mp4

3) use flash as a fall back ; I have done this in the past and it works beautifully

4) Google it !