Try View -> Message body as -> Original HTML

All I see is something like the following (I copied some as an example):

<title>CMA HL NYK ZIM IMP</title>

I have studied your question some more. Does the attached file have an extension of html or htm?

When you click on the filename of the attachment, do you get a small screen asking how to open it? If so, the 'Open with' option allows selection of Firefox. Does that open it properly?

In your system settings, you can designate what program opens each type of file (based on extension).

The extension is html.

I have tried 'Open with' IE and Firefox both and I still get the same results.

Just a thought Look at tools/ alternativ / attachment Is html dokument registered in there? and can you switch t : use FF

If not try to do some action and choose option ".... don't ask, do this always" (Or what it is in english) No you might change as suggested.

In my previous post I stated I tried it with Firefox and it still didn't work.

TB is seeing my html attachment as a document and is opening it Notepad.

Are you using Windows? If so, go to Control Panel -> Programs -> Default Programs -> Set Associations and scroll down to .html; double click it and choose Firefox.

Other OS's have a similar function. If you use Ubuntu, I can find that one for you also.

One more thought: look at Preferences -> Attachments -> Incoming to see if 'HTML document' is listed under content type and the action is 'Use Firefox'. It is possible that the OS is overriding TB in what to do. Since you mentioned IE, I assume you are using Windows.

All the settings you suggested have been tried. It still opens the attachment in Notepad. This problem wasn't present when I used Outlook so I can only surmise it's a Thunderbird problem.

When the attachment is saved and opened, it works fine. It just won't open from within the email.

I know it's one or two extra steps to get it to open correctly but that still doesn't fix the problem.

The mail is probably generated by a script on a web site or router written by an ignorant programmer, quite probably using a script they copied from somewhere else because they know nothing of email.

If you look in your message source (ctrl+U) and check the encoding of the data and I am guessing your will not see.

Content-Type: text/html; charset=UTF-8;
 name="DocumentName.html" 

I would expect something like application octet stream if it is as poorly put together as most script generated emails are.

You may find the add-on Open attachments by extension will work around the deficiencies in the mailers software. But ideally the generating software should be brought up to standard.