Firefox
The web browser Mozilla Firefox, as of version 3.0.1, does not include direct support for saving or opening web pages as MHTML files. Though there is already source code available for viewing MHTML files within the Thunderbird project, this is filed as unsolved bug 18764 within the Firefox project since 1999.
There is an extension which adds MHTML support:
UnMHT. This works in all versions of Firefox from 1.0 to the current one (as of 3.0.1). It is available under the MPL 1.1/GPL 2.0/LGPL 2.1 licence. The current version (v4.7.4) can write and open MHT files. It includes English and German locales. MHT files exported from MS Office (e.g. PowerPoint) are not supported.
For Firefox 1.5, this functionality can be obtained on Windows and Linux operating systems by installing a freely available third-party XPI file from Mozilla Archive Format extension, though the .mht files it generates are not fully compatible with Microsoft's products.