Looking for a good online trading site

I'm looking for a good online trading site. I've heard of a few like icicidirect, kotaksecurities, sharekhan and 5paisa. Unfortunately, most of them seem to like only IE as a browser as clearly mentioned in their site. Now we don't use Windows at office where most of my weekdays are spent, and from where I'd have to do the trading. Are there any good sites which go beyond Windows IE? I'm specifically looking for one which can be used with Firefox.

Looking at the Gateway account info at kotaksecurities, they ask for a margin of 20,000. Does it mean I've to invest at least 20,000 to begin with? I didn't get any info there about what it means.
20k is a bit too much for trading for a newbie like me. I am looking to start with an amount below 5,000 to learn the tricks of the trade. If anyone has knowledge and experience with a good online trading site, I'd like to know more. Thanks!