MHT archives are not that complex, they're just MIME envelopes combining the HTML file with the resources (images etc) that it needs. MIME envelope format is well documented in internet standards. Right now Firefox doesn't support MHT but there's some active feature requests to support it, so maybe it will be added sometime. At worst though, it's not hard to write a script that expands an MHT into a directory of separate files that Firefox can read. The closed web format I really hate is Flash animations, since they are all over the place now, and it's sometimes impossible to navigate past them without the plug-in.