Check Extensions and Plugins

Check Extensions and Plugins

Some extensions can cause problems with logging in to websites, especially extensions associated with certain websites. For example:

• If you cannot log in to Yahoo! and you use a Yahoo! Toolbar extension, try disabling it using the instructions in theTroubleshooting extensions and themes article, restarting Firefox and then following the steps above to clear cookies and cache again.
• If you cannot log in to Gmail and you use a Google extension, try disabling it and then following the above steps to clear cookies and cache before trying to log in again.
• Other extensions may also cause login problems. To determine if they are the cause, see: Troubleshooting extensions and themes.

Some plugins can also cause problems logging in to websites. See the Troubleshooting plugins article for instructions on how to determine whether a plugin is causing your problem.

Help by http://support.mozilla.com/en-US/kb/Cannot%20log%20in%20to%20websites?s=delete+cookies&as=s