Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0
This looks like an installation problem. 'zless' is a wrapper around
'less', and won't work on platforms that don't have 'less'. There's not
much that gzip can do about this, other than have 'zless'-related tests
fail (which is what is happening here).