.mht stands for MHTML (MIME HTML). Internet Explorer uses this to save a web page and dependent files into one file.
To open these in Ubuntu, you need a special program. The Wikipedia entry lists a few:
Mozilla Firefox needs UnMHT or Mozilla
Archive Format extensions to open and
write MHT files. These add-ons work
with Firefox 1.0 and later. Since
version 5.0.0, UnMHT offers partial
support for MHT files exported from
Microsoft Office applications. Mozilla
Archive Format, as well as opening and
saving .mht files, also can save to
its own "MAFF" format. The MHT files
it generates are not fully compatible
with Microsoft's products.
Currently Firefox doesn't include
support for saving or opening web
pages as MHTML files without either of
these add-ons, as of version 3.5.2.
There is, however, source code
available for viewing MHTML files
within the related Thunderbird
project, so in the future it may
become available without such add-ons.
This is filed as an unsolved issue
within the Firefox project since 1999
(see bug 18764) though progress in
fixing it seems to have stalled.
UnMHT is available here.