Exit Firefox completely, open the Windows Control Panel and then,
On Windows 7/Vista, go to "Programs -> Programs and Features", select Mozilla Firefox <version> and click click Uninstall. [3] [4]
On Windows XP (or below), double-click "Add or Remove Programs" Select "Mozilla Firefox <version>" in the list of installed programs and click "Remove" to run the uninstaller.
Starting in Firefox 3, the uninstaller includes the option, "Remove my Firefox personal data and customizations". This will also remove your Firefox user profile data (bookmarks, passwords, cookies, extensions, preferences, etc.). DO NOT select this option if you want to keep your Firefox profile data and settings.
Even if the uninstall fails, as it may in some cases, continue on with the rest of the uninstall instructions.
Delete the Firefox Installation directory located here, by default: C:\Program files\Mozilla Firefox or, on 64-bit Windows, C:\Program Files (x86)\Mozilla Firefox (Optionally make a backup copy of the "plugins" subfolder first, so that you can restore it later if you plan on reinstalling Firefox.)
(Optional) Enable viewing or searching hidden files and folders (details here) then delete these additional files or folders (recommended if you are preparing for a clean reinstall):
The Firefox folder that contains temporary data ( *.mfl files, Cache and Updates), located here: Important: Make sure you delete the Firefox folder under Local Settings or Local. Deleting the wrong folder will remove your user profile data!
Windows 2000/XP: C:\Documents and Settings\<username>\Local Settings\Application Data\Mozilla\Firefox
Windows 7/Vista: C:\Users\<username>\AppData\Local\Mozilla\Firefox
Windows 7/Vista: The C:\Users\<username>\AppData\Local\VirtualStore\Program Files\Mozilla Firefox folder [5] (if it exists)
Windows XP and above: Delete all C:\WINDOWS\Prefetch\FIREFOX* files
If you installed the Quality Feedback Agent component (aka Talkback) in Firefox 2 or below, open the %APPDATA%\Talkback\MozillaOrg\ folder and delete any "Firefox <number>" folders found (e.g., "Firefox10" "Firefox15" and "Firefox2").
If necessary, remove the "Mozilla Firefox" shortcuts in the Windows Start -> Programs menu by right-clicking and deleting the entries. This step should not be needed unless the Firefox uninstall from the Windows Control Panel failed.
The Firefox uninstall will leave behind some Windows registry entries, which can be cleaned up using Windows regedit or a 3rd party registry cleaner. Normally, these extra entries are harmless, and it is not necessary to remove them. Note: registry editing is a potentially hazardous undertaking!.
You may also need to manually change some Windows file associations or icons, even after setting a new default browser, as reported here.
Eso fue escrito originalmente para FireFox, pero aparentemente también funciona para Thunderbird.
Basicamente, después de desinstalarlo tenes que borrar el folder C:\Program files\Mozilla Thunderbird ó C:\Archivos de Programa\Mozilla Thunderbird;
luego tenes que borrar C:\Users\<Nombre de Usuario>\AppData\Local\Mozilla\Thunderbird y, si existe, C:\Users\<Nombre de Usuario>\AppData\Local\VirtualStore\Program Files\Thunderbird.
Suerte con eso
