Upgrading - Thunderbird
From MozillaZine Knowledge Base
Before upgrading to a newer version of Thunderbird, make sure to read the release notes for any comments about installing or upgrading. Version 1.5, for example, is unusual in that it wants you to install into an empty directory (folder) when upgrading. Normally you can rely upon the Thunderbird setup program to delete the older version's (program, not personal) contents if you're upgrading into the same directory (folder) as the older version.
When you upgrade, the new version of Thunderbird should automatically detect your existing profile(s), containing your mail, address books, and settings, and start up with these intact, just as they were before. Because profiles are by default stored completely separately from the Thunderbird program installation directory, they should not be deleted or otherwise affected when you uninstall an old version of Thunderbird or install a new version. However, to be on the safe side, you should make a temporary backup of your profile(s) before upgrading.
There is more than one way you can upgrade. Some users, for instance, might choose to keep an older version of Thunderbird on their computer even after they install a newer one so that they can fall back on the older version if they wish. Below are upgrade procedures that should work well for general users.
Windows (installer version)
Linux (tar.gz version)
Hint to newbies: To avoid (mis)typing long file names, like Thunderbird's *.tar.gz file, merely type the first two or three letters, then press your tab key. The terminal program will fill the rest out for you.
Note: See Moving from Windows to Linux for information on how to use the package managers coming with many distributions for installing or upgrading your Thunderbird installation.
Mac OS X
The common problems depend on which version you are trying to install.
For information about common problems installing Thunderbird 2, see: Thunderbird 2.0 installation issues
Most problems when upgrading to v1.5 are due to not deleting the contents of the Thunderbird program directory before installing the new version in the same location. Delete the contents of the Thunderbird program directory (after uninstalling Thunderbird using the control panel's add/remove programs), download a fresh copy of the setup program (in case it was really a corrupted download) and install it in the same directory. If you run into problems, try running Thunderbird in safe mode to temporarily disable any extensions/themes you may have added. Thunderbird tries to disable any incompatible extensions/themes, but not every extension/theme is well behaved. If that works, then you can try to identify the bad extension/theme and either uninstall or upgrade it.
If you can't get the Windows setup program to run, delete the contents of the Thunderbird program directory (after uninstalling Thunderbird using the control panel's add/remove programs) and download the appropriate win32.zip file. Note that this is a test build for developers, not the official released version, so it might differ from the released version in some ways. Expand it (specifying full pathnames) in the same Thunderbird program directory. You should be able to run that Thunderbird.exe. It should reuse the old registry entries for SimpleMAPI support and mailto: URLs which pointed to that directory. If not, try setting Thunderbird as the default email client.