WOW.... Lots of questions... Hopefully I can help a little bit... I will try to be as accurate as possible, just note that every program is a little different in how it installs.
1) You would direct a download the same as in Win98. If you don't select the Desktop, you will download the file into your /home/<username> directory.
2) It is a tar.gz file when it has that extension. You have to unzip and then untar it before install. The *EASIEST* way is to grep the commands together like: shell> # tar -xvfz location/to/file/installer.tar.gz
3) man tar (or tar --help) for more options -- x (uncompress), v (verbose), f (file), z (unzip)
4)You do not have to uninstall it. You can if you want, but there is no real "uninstall." Just delete the old Mozilla directory. ** TIP ** Firefox is easier and runs nicer than Mozilla IMHO. You can also find installation instructions on their website.
Hope this helps!