My wife was having the same issue, but I could log into my desktop and attach photos with no trouble, so I suspected that something had gotten corrupted in her personal Firefox settings.

Before wiping out her personal profile, I first backed up her bookmarks by selecting Bookmarks -> Organize Bookmarks -> Import and Backup -> Export HTML and saved a backup copy on the desktop.

To delete her profile, I exited Firefox. Next, I right-clicked the Start button, chose "Explore", opened the "Application Data" folder under her ID, highlighted "Mozilla", and deleted it. I then restarted Firefox so that it would create a new profile. The next time she tried attaching a file in Yahoo! Mail, it still complained of the internal error, but when she clicked the "Retry" button, it completed successfully. She tried it several times afterward, and it kept working.

NOTE: to re-import your bookmarks, use the same basic menu picks: Bookmarks -> Organize Bookmarks -> Import and Backup -> Import HTML

Hope this works for you, too!

I see that the above posted image file has embedded spaces in the name.

Does it happen with all files or only with file names that contain spaces?

It is usually better to avoid embedded spaces and convert them to underscores (_).

With Firefox version 5, check that you have NOT blocked "Plugin Container for Firefox". That was why I could NOT send attachments. When I changed it to Permit All I was able to send attachments again. Hope this helps.

You guys are not going to believe this I figured out the problem it's really dumb...ok here goes the attachment uploader loads seperate so I got to thinking it's a popup so I went to options content and under block pop ups there is an option exceptions click on that and in the box type www.firefox.com and hit allow pow its fixed!!!!! :-)

You don't have to disable any addons or extentions!!!You guys are not going to believe this I figured out the problem it's really dumb...ok here goes the attachment uploader loads seperate so I got to thinking it's a popup so I went to options content and under block pop ups there is an option exceptions click on that and in the box type www.firefox.com and hit allow pow its fixed!!!!! :-)