WAS: Ubuntu 12.04 LTS..etc... now Thunderbird cant be killed

[Roger]

Diverging from the increasing number of posts on this desktop somewhat 
repetitive 'removing Unity' topic.
I reference a somewhat more pressing (for me)  problem with Unity and 
Thunderbird

Precise 12.04 with latest updates.
Once Thunderbird is started in Unity more than once over a couple of 
hours it cannot be killed, at least on my machine, and it eats up memory 
and cpu time. My only option is to open a terminal and sudo shutdown now.

I shut down Thunderbird by clicking on the (X) and find it cannot be 
restarted but produces an error message that Thunderbird is running, 
stop the process or restart the machine.
ps aux shows
3758 10.0  4.4 521304 92752 ?        Sl   08:15   0:55 
/usr/lib/thunderbird/thunderbird-bin
sudo kill 3758 has no effect
After 3 hours the pc has slowed to a crawl and has to be restarted.

I have never experienced this with any previous version of thunderbird.
Has anyone else noticed this?
Is there any other way to kill the app other than the above.
Roger