Have you tried saving it then opening it via your Windows Explorer. I know its long winded.

Thanks for the prompt response. Why and how would I open a document in (I use) Firefox when it comes in on Thunderbird? I can't follow this. I tried setting "Microsoft Excel documents" in Firefox Options/Applications to "Open in "MS Excel", but no luck. (I don't understand why it would?)

I'm assuming it has something to do with an update of something, but have no idea what.

It could be an update, FF has been a problem for a lot of people for a few days since the last update. I have never had to open an excel document from TB. I am not sure why you would open excel documents via FF either. Have you tried opening them in excel?

No idea why you are trying to use Firefox - it is a browser.

Please try the following and supply some additional information.

Save the document to a suitable folder on your computer - not in Thunderbird. Then try to open that saved file using the appropriate program - MS Excel Q: Can you open it ok?

Q: What version of Thunderbird are you using? Q: What Operating system are you using? Q: What version of Excel is on your computer ? Q: What is the extension on the attached excel file ? Q: What is the current setting you have in thunderbird on how to deal with excel files? 'Tools' > 'Options' > 'Attachments' > 'Incoming' tab Under Content Type, look for Microsoft Office Excel 2007 Workbook Q: What version of Excel does it say eg: 2003, 2007, 2010 etc Q: what Action does it say is applied ? Please post an image of this window.

Q: What Anti-Virus do you use? Q: Do you scan incoming mail ?

Thanks johnmwc2654 and Toad-Hall. I'm not trying to open an Excel document in Firefox, it was a suggestion. I have tried opening the documents (several times, several documents) direct to Excel (MS Office Suite 2010) and saving it as a separate file. Nothing works, same error message. I am using Windows 10, xls and xlsx files won't open. On TB settings for all Excel docs, it's "Use MS Excel" (has not been changed). I use Norman AV, yes, I scan the mail, nothing has been changed in Norman. I even scanned the system just to be sure. As noted, I can open all other documents.

Just for the record and I'm not sure if it matters - I use the Dutch version of all mentioned programs - and have for years without problems. I also can open the docs on my phone and tablet, so it's something with the PC.

Pfeifle said

Thanks johnmwc2654 and Toad-Hall. I'm not trying to open an Excel document in Firefox, it was a suggestion.

A very bad suggestion I would say. You could use Excel if you have it, LibreOffice as a free substitute if you do not. You could also install the Excel viewer provided by Microsoft. But Firefox has none of the file descriptions necessary to even start to open an Excel file. As Toad-Hall says, " it is a browser."

I have tried opening the documents (several times, several documents) direct to Excel (MS Office Suite 2010) and saving it as a separate file. Nothing works, same error message.

To go back toJohns comment Right click the attachment and select save as. Save the file to your desktop. Double click on the saved file. Does it open?

I am using Windows 10, xls and xlsx files won't open. On TB settings for all Excel docs, it's "Use MS Excel" (has not been changed). I use Norman AV, yes, I scan the mail, nothing has been changed in Norman.

That is probably the cause of your problem. it is corrupting the attachment while it is scanning it on arrival. Disable the email scanner get someone to send you another attachment and see if it opens when you get it.

I even scanned the system just to be sure. As noted, I can open all other documents.

All the more reason to be looking at the anti virus program.

Just for the record and I'm not sure if it matters - I use the Dutch version of all mentioned programs - and have for years without problems. I also can open the docs on my phone and tablet, so it's something with the PC.

The Language used should not be relevant in this case. But the PC has Norman and your other devices probably do not. Just another reason to look at it.

Excel can be picky about file extensions. I may claim an XLS-format sheet with an .xlsx extension is corrupted, or vice versa. You might try renaming the file extension to see whether that is the problem, although it would be very strange for that to be the problem in every case.

Thanks folks for all the suggestions, but nothing really helped, including changing the extensions, stopping the antivirus program, etc.

What did help, but don't ask me why was, not doing anything about it for a while (too busy) and a restart; that seemed to clear everything up. Of course I had done this many times in between, but the problem has disappeared, seemingly spontaneously!

The wonders of electronics.

Charles