Firefox does not care what web-site you are accessing. One thing to check are the settings on that site. Some have an option for additional login security.

Many site issues can be caused by corrupt cookies or cache.

• Clear the Cache and

• Remove Cookies
  Warning ! ! This will log you out of sites you're logged in to.

Type about:preferences<Enter> in the address bar.

• Cookies; Select Privacy. Under History, select Firefox will Use Custom Settings. Press the button on the right side called Show Cookies. Use the search bar to look for the site. Note; There may be more than one entry. Remove All of them.
• Cache; Select Advanced > Network. Across from Cached Web Content, Press Clear Now.

If there is still a problem, Start Firefox in Safe Mode {web link} A small dialog should appear. Click Start In Safe Mode (not Refresh). While you are in safe mode;

Type about:preferences#advanced<Enter> in the address bar.

Under Advanced, Select General. Look for and turn off Use Hardware Acceleration.

Poke around safe websites. Are there any problems?

Then restart.

Yahoo recently added the option for two-factor authentication where it sends you a code and then you enter it and Yahoo sets a special cookie on your browser so it can recognize that you have performed this step with your current browser the next time you sign in.

Fred's suggestion to clear your current Yahoo cookies should take care of any corrupted cookie, but if you have Firefox set to flush cookies between sessions, you'll continue to have the same problem.

What is your standard cookie policy? You can review that on the Preferences page:

"3-bar" menu button (or Firefox menu) > Preferences

In the left column, click Privacy. Then on the right, if the selector is set to "Firefox will: Remember history" you can change that to "Firefox will: Use custom settings for history" to view detailed settings.

Possible cookie policies:

• Block all cookies -- should cause this problem
• Allow session cookies ("Keep until: I close Firefox") -- should cause this problem
• Allow persistent cookies ("Keep until: they expire") -- should work

If you only allow session cookies, try adding an exception to allow persistent cookies for yahoo.com. You can use the Exceptions button and type or paste https://yahoo.com in that dialog, or you can use the Page Info dialog while you're on a Yahoo site.

You can call that up using any of these:

• right-click (on Mac Ctrl+click) a blank area of the page and choose View Page Info > Permissions
• (menu bar) Tools menu > Page Info > Permissions
• click the padlock or "i" icon to the left of the site address, then the ">" icon, then More Information > Permissions

Scroll down to "Set Cookies" and uncheck the "Use default" box, and then select "Allow". That should be saved immediately, so you can just close this dialog. However, it only applies to new cookies, so you may have the problem one more time with the existing session cookie.

While you have the Preferences page handy, do you have Firefox set to clear cookies when you close the browser? That overrides exceptions about which sites can set cookies -- that setting clears ALL cookies.

You can double check that on the Preferences page, Privacy panel: If you have a checkmark for "Clear history when I close Firefox", click the Settings button to the right and make sure that Cookies are NOT selected.

If you do not have Firefox set to clear cookies at shutdown, some other ways the cookie might get cleared are:

• An extension that manages cookies clears them at shutdown or statup
• An external program cleans Firefox data
• Your cookies.sqlite database file becomes locked or corrupted

We can walk through diagnosing those things if needed.

Sorry, are you saying you have this problem in Safari or in Firefox or both? I got a little confused toward the end of your question. My reply was for Firefox, but the same principle applies to every browser: if the site requires a cookie to know you, and you cleared it, you have to prove your identity again from scratch.