Exact Duplicate: https://stackoverflow.com/questions/473465/firefox-plugin-to-simulate-slow-internet-connection
How do I simulate a slow internet connec
Not that I know of. If you want to see how things look when they're loading you can use firebug and set breakpoints in the javascript, or breakpoints on your server. I always use this technique to see how things look when they're loading.
Another option is NetLimiter, I've heard it works but haven't tried it myself http://www.netlimiter.com/