I wanted to install Mozilla Firefox 2.0.0.5 Linux version on my Ubuntu 6.06. Sadly, when I used Terminal using the command:
apt-get install firefox-2.0.0.5.tar.gz, it can "read" the package on the root account but it tells me that it cant be found. I have downloaded it into the root file.... so much so that I had even forgotten where did I put all of my files .(Because I just copy paste, Copy paste due to the failure of installing.)
What should I do?

1) apt-get is for installing packages from a package reposity; in your case it means you use it to install a package from the Ubuntu internet reposity. If you have Firefox installed already (Ubuntu package), it would have been updated already if there was 2.0.0.5 version. Correct usage:
2) To install local binary packages, you use dpkg; for example to install firefox.deb you would use
(note: if firefox.deb isn't in the current directory, write full path!)

3) .tar.gz is a Gzip-compressed tar archive. Sort of like zip-file, but in another format. The same way .tar.bz2 or .tar.Z are. A .gz file is a Gzip compressed file, and a .tar file is an archive (several files in one file, but no compression) - therefore .tar.gz is a compressed archive of several files. You don't install them with apt-get or dpkg or anything like that; Slackware uses .tar.gz packaging system, but still it needs that the package is really a Slackware package (in that case); the Firefox package you've downloaded from the net is probably not an installable file as such, but a compressed archive that includes the files to use. Correct way:
this would extract firefox-2.0.0.5.tar.gz into the current directory (hopefully creating a subdirectory, or else the files are a mess); in the tar options 'x' stands for eXtract, 'z' stands for gZip format and 'f' stands for File (after which you type the filename to extract). So 'xzf' means "extract gzipped file ....."

After this you can go into the directory that was extracted, and try launching Firefox - on some systems it works directly from there (look for somekind of binary, maybe "firefox" or "firefox-bin"). If it doesn't work (start from command line - see the errors) it could be that the library files can't be found. The libraries are inside the directory where the binary is, but your system might not look from there. Solution is to either tell the system to look from that directory also, or to copy the libraries to a directory where the system finds them. There should be directions for the "installation" (that means COPYING THE FILES TO CORRECT PLACES, as the Firefox you download is not something you "install", but something you "just run") inside the directory you extracted: look for files called README and INSTALL. Or inside doc/ directory.

Because Ubuntu is a binary distribution, I don't recommend getting software by downloading a .tar.gz and messing around with it, especially if you're unsure how to do it. Ubuntu offers Firefox trough it's reposities, you can install it with Synaptic, and it's automatically upgraded (or you're asked if you'd like to do it) when it is in the reposities. If you get the same program from a .tar.gz file, Ubuntu's package manager is not aware of it, and cannot upgrade it - if you overwrite the files of the existing Firefox install, the package manager might get confused about that. Best way to either stay with the Ubuntu version, or locate a .deb file that is for Ubuntu specifically from the web and install that, or if you want to use the .tar.gz version, remove the Ubuntu's Firefox package trough Synaptic and then start using the .tar.gz-extracted one.

apt-get is for installing a .deb package---I don't think it knows what to do with a .tar.gz format. (And doesn't it just work with repositories?)

Just do tar -xzvf firefox*
cd into the folder that this creates and run the install script.

I tried to remove the package from synaptic and reinstall again. Success, but when I tried to open from Applications-> Internet-> Mozilla Firefox,it gave me an error.
Error code as follows:
Could not launch menu item

Details: Failed to execute child process "firefox" (No such file or directory)

And gave me an option. "OK" ,click the ok button and the thing would quit?
Why??!! Windows is A LOT BETTER in terms of installing software. I hope Windows Installer would be ported into Mac/Linux and other Unix sytems for the benefit of the world.