Why is it that some packages install in the normal location (i.e. /usr/bin or whatever) while some such as Mozilla and KDE install in the opt folder?
There isn't a hard-and-fast rule about which places applications should go.
First, some go into the /usr directories out of tradition (based on the rationale: that's where they've been put traditionally and many users expect them to be there).
However, there is a strong preference to place large and complex applications and environments such as Mozilla and OpenOffice.org in /opt ... and relatively small-to-medium-sized applications tend to go into the /usr directories.
And some distributions don't use /opt much for even large and complex applications.
[Personally, I would like to see more applications put into /opt because this concentrates and localizes the application code, data, and configuration files; and make, for me, at least, it easier to understand and maintain. We're all familiar with the sometimes aggravating process of tracking down configuration and data files that are peppered about the file system.]