"Memory leaks" in Firefox (Most of the time they're not but I won't get into that) aren't a problem if you have a good amount of memory. Especially if you regulate how much maximum memory Firefox can use.
It has an immense amount of features; many of which I haven't seen on other free IM clients. It's great. Their phone rates are good too.
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It's very good for an alpha release, very stable. I don't recommend it as your main version of Firefox to use (as said before, stick to 1.5), but you could download it on a separate drive just to play around with.
