I receive emails with attachments ending in .eml which I can't open.

Files with the .eml extension often originate in Outlook Express or another Microsoft email client and can be read by the same software. I believe Outlook Express was included in Windows XP. Not the best mail client, but perhaps tolerable for this usage?

To associate .eml with Outlook Express, you could try using

right-click > Open With

You might need to hold down the Shift key when right-clicking in order to get that item; I have forgotten some details about XP...

Also, here's a Microsoft help article with more ideas: https://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/associate_file_with_program.mspx

As I said, I am not computer savey. I really don't understand what you are trying to tell me to do. Where do I start?

.eml files are email type files. Firefox doesn't do email. Therefore this question is off-topic for a Firefox support forum.

If you can't follow the Microsoft topic that Jefferson linked, try posting in a Windows support forum, like here: http://windowssecrets.com/forums/

Hi chuckiebaby, I should have said I meant to right-click the downloaded file in Windows Explorer.

After you download a file in Firefox, it should appear on the panel that opens when you click the download arrow on the toolbar. Or you can show all Downloads (press Ctrl+j). The file should have an icon on the right side to "Open Containing Folder". Click that, and Windows should open a Windows Explorer window with the download highlighted. Then right-click the file and look for Open With.

From there, you are in the land of Windows and if you need further assistance, I agree with the-edmeister's suggestion.