Thunderbird doesn't look for profiles in the default location where profile folders are stored. Instead, it uses a <b>profiles.ini</b> file to keep track of the profiles it knows exist and where the corresponding profile folders are located. You're getting this error because Thunderbird cannot find the profile folder that <b>profiles.ini</b> is telling it to use. This could be because something bad happened to either the profile folder or to <b>profiles.ini</b>. I don't see how this could be related to what you said you did, but you may look at the contents of <b>profiles.ini</b> with a text editor in case there is something there that could shed some light on what happened. Since you only seem to have a single profile, you may be able to fix the problem as follows: # Quit Thunderbird if it's running. # Take note of how the profile folder you have is named, because a new one will be created alongside it, and you'll need to differentiate between the two. # Either delete <b>profiles.ini</b>, or rename it to something else, e.g. <b>profiles.bak</b>, if you want to keep it so you may try to figure what happened. # Launch Thunderbird. Instead of looking for a profile folder that doesn't exist, it will create a new <b>profiles.ini</b> file with a new default profile and will let you proceed. # You will be asked to set up a mail account, but you don't have to. You may cancel the account setup process and choose the option to <b>Use Thunderbird without an email account</b> when asked for confirmation. # Choose <b>Help > Troubleshooting Information</b> from the menu bar. # Click <b>about:profiles</b> in that page to open the <b>About Profiles</b> built-in Profile Manager. # <b>Create a New Profile</b>, read the introductory text if this is new to you, then click <b>Continue</b>. # Click <b>Choose Folder</b> and choose the profile folder you had there at the beginning. # Name the profile however you want and click <b>Done</b>. You have now associated the profile folder with an actual profile that Thunderbird knows about. # The new profile may have already been set as default. If not, use the <b>Set as default</b> button under it to make it so. # Click <b>Launch Profile</b> under the new profile to start using it.