I don't know whether this problem is continuing but Yahoo has a help article about that error code: Help Central | - SLN3543 - Temporary Error 14. They suggest clearing your cache and cookies. Let me suggest these alternate steps for that:

(1) Clearing Firefox's Cache

"3-bar" menu button (or Tools menu) > Options > Advanced

On the Network mini-tab > Cached Web Content : "Clear Now"

If you have a large hard drive, this might take a few minutes.

(See: How to clear the Firefox cache for additional details if needed)

(2) Remove the site's cookies without affecting other sites. While viewing a page somewhere on yahoo.com, try either:

• right-click and choose View Page Info > Security > "View Cookies"
• (menu bar) Tools > Page Info > Security > "View Cookies"
• click the padlock or globe icon in the address bar > More Information > "View Cookies"

In the dialog that opens, you can remove the site's cookies individually.

Then try reloading mail and logging back in. Does that help?

The problem persists. I deleted all cookies and had to log in again. The error message returns and attachment again failed.

Hmm, I don't know what else could cause that error code. Could you test in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement?