Interestingly enough, on Firefox both versions try to download. I think this is happening because the server configuration was set up incorrectly. I think the configuration was modified to parse .html files as .php, but the content type is not getting changed correctly when the PHP parser processes the document.

Try adding the following:

AddHandler x-httpd-php .html

The reason is that there are two different ways Apache can process PHP files, depending on the way PHP is installed on the server, and the way the Apache server was compiled. It has to do with how Apache sends the call to the PHP processor. All you need to know is when AddType fails, use AddHandler.

If this fails, on some servers you need to create a seperate line for each extension. So instead of the line you have above, you would use:
AddType application/x-httpd-php .html
AddType application/x-httpd-php .htm

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