Will I download Firefox? Probably. Setting records is fun.
Is there a good reason not to? Um, yeah, if you like Java and run Ubuntu. Firefox's Java JRE module sucks; a major drawback, and Ubuntu, being Linux-based, can run Java all day, but won't allow cross-platform calls, so no JRE-JDK crossbreeds. It's a known flaw and has been since Firefox 2.0.0.9 and Java 6, but Mozilla hasn't fixed it yet. There are some workarounds, but I still have problems on heavy Java portals with Firefox freezing solid and requiring a kill.
And no, Seamonkey and Iceweasel have not fixed the problem either. Mozilla is supposed to be reconfiguring their JVM to handle the JRE 6 and up calls, but even Java 5 didn't play nicely and Sun no longer supports 4 or below. It's a mess, but I do have hope that things will improve.