I cannot seem to download files. Terminal always says file not found even though it exists

Hi all, I'm a total beginner with linux distros and I booted Zenwalk from a flash drive today and have tried downloading and installing the latest version of firefox. I managed to download the .txz file but whenever it comes to installing it from the terminal or pkgtool, it says "Cannot install mozilla-firefox-66.0-x86_64-190319.txz: file not found"
This happens if and when I use the "installpkg" command or even the "#upgradepkg -install-new...."

Anybody know a fix?
Thank you