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If you find that you can't log in to a website that requires a username and password, follow the steps described in this article.

If you can log in to a website manually, but can't save or use saved usernames and passwords, see Usernames and passwords are not saved.

Enable cookies for the website

Make sure that cookies are enabled for the site in question:

At the top of the Firefox window, click on the Firefox button and then select Options.At the top of the Firefox window, click on the Tools menu and then select Options.On the menu bar, click on the Firefox menu and select Preferences....At the top of the Firefox window, click on the Edit menu and select Preferences.
Click the menu button and choose Options.Preferences.
Select the Privacy panel.
Set Firefox will: to Use custom settings for history.
Make sure Accept cookies from sites is check marked.
Make sure Accept third party cookies is check marked.
- Disabling third party cookies can cause problems with some websites. Enable this setting to rule it out as a cause for your problem.
Click Exceptions….
Make sure the site you're trying to access isn't listed.
- If it is listed, click on its entry, then click Remove Site.
Click OK to close the Options window.Click Close to close the Preferences window.Close the Preferences window. Close the about:preferences page. Any changes you've made will automatically be saved.

At the top of the Firefox window, click on the Firefox button and then select Options.At the top of the Firefox window, click on the Tools menu and then select Options.On the menu bar, click on the Firefox menu and select Preferences....At the top of the Firefox window, click on the Edit menu and select Preferences.
Click the menu button and choose Options.Preferences.
Select the Privacy panel.
Set Firefox will: to Use custom settings for history.
Make sure Accept cookies from sites is check marked.
Make sure Accept third party cookies is set to Always.
- Disabling third party cookies can cause problems with some websites. Always accept third party cookies to rule it out as a cause for your problem.
Click Exceptions….
Make sure the site you're trying to access isn't listed.
- If it is listed, click on its entry, then click Remove Site.
Click OK to close the Options window.Click Close to close the Preferences window.Close the Preferences window. Close the about:preferences page. Any changes you've made will automatically be saved.

If you changed any settings, try logging in to the site again.

Clear history for that site

The cookies and temporary data already stored on your computer may be causing the problem. Delete them, then test for your problem:

At the top of the Firefox window, click the Firefox button, go over to the History menu and select Show All History to open the Library window.On the menu bar, click the History menu and select Show All History to open the Library window.At the top of the Firefox window, click the History menu and select Show All History to open the Library window.
Click the menu button , choose History and then click the Show All History link at the bottom of the list to open the Library window.
At the top right corner, type the name of the website you wish to forget in the Search History field, and press EnterReturn.
In the resulting list, right-clickhold down the Ctrl key while you click on the site you wish to forget, and select Forget About This Site.
Close the Library window.

Clear all cookies and the cache

If deleting cookies for the site with the problem did not fix the error, clear all the cookies stored on your computer and clear the Firefox cache:

At the top of the Firefox window, click on the Firefox button, go over to the History menu and select Clear Recent History....On the menu bar, click on the Tools menu, and select Clear Recent History....At the top of the Firefox window, click on the Tools menu, and select Clear Recent History....
In the Time Range to clear: drop-down, select Everything.
Click the arrow next to Details to display the list of items that can be cleared.
Select both Cookies and Cache.
Click Clear Now.

At the top of the Firefox window, click on the Firefox button, go over to the History menu and select Clear Recent History....On the menu bar, click on the History menu, and select Clear Recent History....At the top of the Firefox window, click on the History menu, and select Clear Recent History....
In the Time Range to clear: drop-down, select Everything.
Click the arrow next to Details to display the list of items that can be cleared.
Select both Cookies and Cache.
Click Clear Now.

Click the menu button , choose History and select Clear Recent History....
In the Time Range to clear: drop-down, select Everything.
Click the arrow next to Details to display the list of items that can be cleared.
Select both Cookies and Cache.
Click Clear Now.

Check Extensions and Plugins

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

If you can't log in to Yahoo! and you use a Yahoo! Toolbar extension, try disabling it using the instructions in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article, restarting Firefox and then following the steps above to clear cookies and cache again.
If you can't 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 Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.

Some plugins can also cause problems logging in to websites. See the Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems article for instructions on how to determine whether a plugin is causing your problem.

Security application is blocking cookies

Some Internet security and privacy applications have features that block cookies. Check the documentation that came with your software or visit the software provider's support site, to see if your application includes a cookie control feature and how you can change those settings.

Remove corrupt cookies file

If you can't log in to websites after removing the cookies associated with it, clearing your Firefox cache, and trying with your extensions and plugins disabled, and have checked security software, you may have a corrupt cookies file in your Firefox profile folder.

This will remove all your stored cookies and essentially log you out of all saved sessions on all websites.

Open your profile folder:
At the top of the Firefox window, click on the Firefox button, go over to the Help menuOn the menu bar, click on the Help menuAt the top of the Firefox window, click on the Help menu and select Troubleshooting Information. The Troubleshooting Information tab will open.Click the menu button , click help and select Troubleshooting Information. The Troubleshooting Information tab will open.
Under the Application Basics section, click on Show FolderShow in FinderOpen Directory. A window with your profile filesfolder will open.

Note: If you are unable to open or use Firefox, follow the instructions in Finding your profile without opening Firefox.

At the top of the Firefox window, click on the Firefox button and then select ExitAt the top of the Firefox window, click on the File menu and then select ExitOn the menu bar, click on the Firefox menu and select Quit FirefoxAt the top of the Firefox window, click on the File menu and select Quit.
Click the menu button and then click ExitQuit .
Delete the cookies.sqlite file and any cookies.sqlite-journal files.
Restart Firefox.

Includes information from Websites report cookies are disabled (mozillaZine KB)

Firefox