This one had me really puzzled. Apart from anything, it appeared to be written in the kind of fluent Hungarian that is favoured by people more technically minded than myself.
It appeared as Thunderbird was trying to automatically upgrade itself. It places the computer in an annoying loop. If you select retry the message re-appears, if you cancel, then you simply repeat the problem at a later date.
It turns out that the usual cause of this error is having a Logitech webcam installed on your system. If you install the Quickcam software and already have Thunderbird installed, it will set Thunderbird as its default associated email client. If Thunderbird is set as your default email program in the Quickcam software, Quickcam will not let Mozilla update the file.
The fix is to disable the webcam by right-clicking the Quickcam icon in the system tray and selecting "Exit") and try again. Once the upgrade is complete, Quickcam can be restored, although the problem seems to be associated with each incremental upgrade of Thunderbird v2, so be prepared to disable Quickcam again.