There are two types of passwords. A primary or master password which is used to lock Thunderbird account password store, and account passwords. If you are needing to reset the primary password these instructions will do it, they will also remove any stored passwords if there is a primary password set. https://support.mozilla.org/en-US/kb/protect-your-thunderbird-passwords-primary-password#w_removing-your-primary-password If it is an account password, Thunderbird has only ever stored what you gave it to store. If you no longer know that password you will need to contact the relevant mail provider to have the account password reset. Please note that post update we often see folks with antivirus products that have lost the plot and can not longer access their mail because the antivirus software is actively blocking them. But as you say this is following a "desktop reset" whatever that is, I guess update issues are not pertinent