You can download and install a replacement for 'pkgtools' from the Tukaani project
and, if you also install Tukaani's LZMA Utils
, you will be able to install .tlz packages.
Please be aware that you might very well experience problems installing TLZ packages designed for use with VL 5.8 on a VL 5.1 system. Your 'ffox.tlz', for example, may require system libraries that are a newer version.
Nonetheless, Tukaani's PKGTOOLS provides some nice additional functionality -- it can setup Slackware mirrors and sites such as LinuxPackages.net to be "repositories" from which you can install packages directly -- and is (to my knowledge) a completely safe alternative. The LZMA Utils are also in and of themselves useful to have available.
There may be VL 5.1 versions of these two packages available. If so, you should probably use them instead of the ones provided by Tukaani.