Caveat: the following answer DOES NOT apply if you downloaded Thunderbird V16 from mozilla.org an installed it manually, if you have done that I would recommend uninstalling it and using the lastest version from the Software Center (all your email and settings should be saved but just in case save
/home/.thunderbird and you can just paste it over the new folder(by the same name) after install if your settings and emails disappear.)
Thunderbird V16 is beta software so you must be using the beta ppa for Thunderbird.
You will either need to disable it or remove it.
The easiest way to disable it is to open the update manager in the "gear" icon of the Unity Bar(it will be the fourth menu item, may also say Updates Available):
click on the Settings button and then Other Software in the window that pops up:
You will then need to uncheck the ppa for
http://ppa.launchpad.net/mozillateam/thunderbird-next/ubuntu then close the window and on the Update Manager window click Check to update your source list
When finished it may show you Thunderbird V15(not yet in the regular Ubuntu Repo) or the current V14 (V15 was just released and should replace V14 shortly so you can just install V14 and then update when the official release comes out) if so you can just install it form there, if it does not show up or gives errors about a newer version being installed, you will need to use the terminal and type
sudo apt-get dist-upgrade and then it should show you V14 or V15 of Thunderbird and allow you to install it from there.
You can also purge the beta ppa of Thunderbird by using
ppa-purge and it will remove the beta ppa and then downgrade Thunderbird in one step. In a terminal type the following:
sudo apt-get install ppa-purge
sudo ppa-purge ppa:mozillateam/thunderbird-next
It will tell you that Thunderbird will be downgraded to V15/V14 and ask if you want to contiue, if you answer yes it will remove the ppa and downgrade Thunderbird to the lastest stable version.
 A side affect of downgrading that I have not encountered with upgrades was encountered by the OP of the question, After a successful downgrade, the account profile was not found, we fixed it with the following:
- Open the Home folder and press
Ctrl+H to show hidden files
- Open the
.thunderbird file & find the file with your profile in it- it will be a file with a random name of letters and/or numbers followed by
3qwerty.default). If there is more than one, open them and see which one has your settings.
- Open the file called
- Edit the last line in the
profiles.ini file to read: "Path=name_of_your_folder.default" (i.e.
Path=3qwerty.default) and then save the file. When you open Thunderbird, your account should be there.