Firefox 3 supports .mht format
Actually Firefox 3 still does not support the.mht format but there is an addon called Unmht that can help to save the webpages in the .mht ( webarchive) Format.
I was using the Mozilla Archive Format is the Firefox 2 but when Firefox 3 was relesed I decided to stay in Firefox 2 since I did not have a supported webarchive addon. ( Also there were some other addons which were not compatible ).
I first found out about this addon on the Digital Inspiration and in this post.

Great tip dude, in my blog i post about firefox too, but, it’s in Portuguese.
Thanks!
Comment by Gullit | July 20, 2008 |
For your information, I’m now maintaining a new version of MAF for Firefox 3 at:
http://maf.mozdev.org/
The latest version is 0.8.0, and supports both the MAFF cross-browser ZIP-based format, and MHT. However, if what you need in particular is MHT support, I still recommend using UnMHT.
For comments or suggestions, please write to the MAF mailing list.
Comment by Paolo Amadini | November 26, 2008 |
This format is not cross-browser yet. If I save page in opera it onens only in opera. If I save page in FF, it opens in FF. The same I see in IE.
Comment by kirill | October 17, 2009 |