Two thoughts, and an obvious third comment.

First, because the browser logo started out as the status indicator in NCSA Mosaic and in the '90s browsers, before UI designers started thinking about screen real estate. The animated logos in these indicators needed to display indefinite progress, so they were elaborate spinners, and spinners are round.

Second, because the globe is a visual metaphor for the world wide web, and circles figure prominently in navigation, which is a design motif for browsers.

Finally, because the first major commercial browsers (Netscape and IE) had circle logos, so circles became part of the marketing vocabulary for browsers. You could do a square logo now, but it'd stick out.

Note that the original Mozilla browser logo was not a circle. AND WHERE IS IT NOW?

I'm guessing it's just the desire to look like the IE logo. Even others are round: Maxthon, Flock, xB Browser. Netscape also went from square to round:

http://en.wikipedia.org/wiki/File:Netscape_classic_logo.png

http://en.wikipedia.org/wiki/File:Netscape.svg

Also, Phoenix/Firebird wasn't round:

http://www.martijndevisser.com/blog/wp-content/uploads/2007/...

Technically, Opera, being the independent sort, is more of an oval.

Good observation. It could be because the "world" in www is round.