Create a new profile as a test to check if your current profile is causing the problems. See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

Profile Backup and Restore

• http://kb.mozillazine.org/Profile_backup
• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Same problem still exists after following the above direction.

Boot the computer in Windows Safe mode with network support (press F8 on the boot screen) as a test to see if that helps.

Problem still exists in safe mode. Can't open attachments on Firefox, Yahoo mail.

hello, does it work when you access yahoo mail through this link?: https://mail.yahoo.com/

^{reference: https://support.mozilla.org/en-US/questions/971042}

I can open now open attachments when opened through that link. I bookmarked by my email through that link. Thanks Phillip. What's different about that link?

I'm not sure as to what exactly is going on, but it looks like yahoo has changed something on their server that makes certain old url formats (that you may have used before as a bookmark or in your history) dysfunctional - at least quite a number of other users have reported that...
so using the generic address https://mail.yahoo.com/ as a bookmark should lead you to the current & working url.