In my installed programs, I have

"Firefox 1.0.1"

and

"Firefox 1.0.2"

Do I need both of these installed or do they build on top of each other?

They don't build on each other, but they probably both installed in the same directory. So, if you uninstalled one, you'd delete the install directory for the other.

So now that 1.0.3 is out, do I need to download and install its setup file to get it updated or is there an integrated update function somewhere?

I never manually installed 1.0.2 to my knowledge, I started out with 1.0.1.

Thanks for the replies, btw.

http://www.mozillazine.org/talkback.html?article=6336

Mozilla Firefox 1.0.3 and Mozilla 1.7.7 Coming
Tuesday April 5th, 2005

New maintenance updates for Mozilla Firefox and the Mozilla Application Suite are on the way. Mozilla Firefox 1.0.3 and Mozilla 1.7.7 will include critical security and stability fixes but no new features. Windows Firefox users will be relieved to hear that Firefox 1.0.3 will fix the bug that causes multiple Firefox entries to appear in the Add or Remove Programs window.

Personally, I'd skip the official installs anyway. The Moox optimized builds are noticibly faster. http://www.moox.ws/tech/mozilla/firefox.htm

I might try that Sanchek after they have a 1.0.3 release version, but for now I'll stick with the official stuff.

When I want to go 1.0.3 official, do I need to completely uninstall my existing version or just install 1.0.3 on top of what I've got?

I might still wait longer since some of my extensions or themes may not yet be compatible with 1.0.3.