Hi there Putput...
good question! It has had me stumped for a few days. There is a common security setting in Outlook Express that stops people from opening attachments - but you should be running fine with Thunderbird...
Now - keep in mind that there are lots of things that could be stopping up the works... but here is a likely suspect after some research on the net...
Thunderbird has a file that keeps a track of what it should do with attachments when you click on them... This file is called "mimeTypes.rdf". Now, it is possible that your recent upgrades have corrupted this file - it has been known to happen before. What that would leave you with is a thunderbird program which has no idea what you are talking about when you click on your "Doc" file!
I would start with the easy stuff - go to your preferences or options file. (Tools / Options I think) and then click on the "Attachments" tab, and click on the "View and Edit" button in the "File Actions" section. This should give you a list of what thunderbird wants to do with various file types. See if you can remove and then add the "doc" type again. This might see you right.
If that doesn't work - you might have to look at a bit of a geek solution. Basically you may need to either manually edit the mimeTypes.rdf file, or remove it completely and let Thunderbird rebuild it for you!
Check out some guides at the following links - feel free to post here again if you want some more advice or pointers
http://www.freeemailtutorials.com/mozillaThunderbird/attachmentsOptions.cwdhttp://kb.mozillazine.org/Actions_for_attachment_file_types