Assuming the mbox files are in either a POP mail account or stored in 'Local Folders' mail account, then they are stored in your 'profile name' folder on your computer. So like anything on a computer, it is always a good idea to backup the profile name folder and store it on eg: external hard drive. The profile name folder is usually called 'xxxxxxxx.default', where the x's are letters and numbers. Updates, effect the Thunderbird Program and not the Thunderbird profiles containing emails. They are stored in entirely different locations. So your mail accounts containing mbox files full of emails should not be effected by any update. On rare occasions you hear of missing mail accounts, but this usually means Thunderbird could not locate the mail account for some unknown reason, so created another blank one. The original is still in the profile and can easily be accessed by pointing the directory to the correct one. I would create a backup and then try an update. Make sure Thunderbird is closed before performing a backup. You can also navigate directly to your profile folder at the following path: C:\Users\<Windows user name>\AppData\Roaming\Thunderbird\Profiles\<Profile name>\ ...or by using Thunderbird menu path Help->Troubleshooting information->click on the "Show Folder" button. The AppData folder is folder is a hidden folder; to show hidden folders, open a Windows Explorer window and choose "Organize → Folder and Search Options → Folder Options → View (tab) → Show hidden files and folders".