.mht significa MHTML (MIME HTML). O Internet Explorer usa isso para salvar uma página da Web e arquivos dependentes em um arquivo.
Para abri-los no Ubuntu, você precisa de um programa especial. A entrada da Wikipedia lista alguns:
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.[3]
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.
O UnMHT é disponível aqui .