I am able to log in to my Steam account using the Steam application, but when using the same username and password through a web browser, the Steam website indicates that my credentials are incorrect. What can I do?
Try disabling any browser extensions or add-ons you have installed. These may interfere with the login process.
For more information on how to remove extensions or add-ons with common browsers please visit the following links:
Mozilla Firefox: http://support.mozilla.org/en-US/kb/disable-or-remove-add-ons
If you are unable to log into your Steam account with your current web browser, please try using a different browser and test the issue.
You may wish to perform a clean install of that web browser, in case any older extensions or addons may still be interfering with this process.