When you start working with Cognos Active Reports you can save a report as a MHT file. To view these MHT files you can use Internet Explorer.
When you don’t want to use IE the alternative is Firefox. To open the MHT file you can use the plugin called UnMHT
You can find the plugin on this page: https://addons.mozilla.org/nl/firefox/addon/unmht/
After clicking on “Add to Firefox” you’ll get a warning
Click “Install” to finish the installation
After that drag a .mht file into the Firefox address bar and see the mht file gets opened properly
I must note that the functionality in Firefox is not the same as in Internet Explorer. The report is not really active.