You need to check with the author of the extension about how to reset the password.

Does it save the password in the Firefox password manager? To check that, in the Tools menu select Options to open the options dialog. In the Options dialog select the Security panel then click the "Saved Passwords" button to open the passwords manager. Click on the "Show Passwords" button to view the passwords (if you have set a master password you will need to enter it). Check the list of saved passwords to see if the one from the chat manager is stored there.

There are no passwords saved.:( What do you mean author of the extension?

The person who wrote the extension, it was not written by Mozilla.

I don't know who wrote the extension. Im not even sure what you are talking about.

Whoever created the Facebook chat manager extension would know where it stores the password and how to reset it. If it is this add-on - Facebook Chat History Manager, the homepage for the extension is http://apps.facebook.com/firechm - if it is that extension you would be better off asking there about this.