I have been using t-bird for a long time and have not encountered such problem.
The error message: Thunderbird.exe-application error - "the instruction at "0x7c911e58" referenced memory at 0x00000000. The memory could not be read. Sometimes a similar message appears but with a different sets of figures: 0x7c910f29 ...at 0x61737365.
What could be the problem. I have recently migrated to FF3.

Its not likely your FF3 is causing the problem.
What version TB are you using?
Have you compacted your mail folders?
Does the error messages appear very often?
Have you tried to run TB in safe mode?

I use TB2.0. I try to solve the problem by disable auto downloading emails (pop3) when TB started. Another thing I notice is the problem happens when downloading emails from Hotmail. (I have following the instructions in one of the TB forums to have the necessary addons for this purpose installed and it had been working fine until a couple of days ago). Could it be due to hotmail settings? I shall try to reinstall auto download of other email accounts except hotmail and see the outcome later.
Thanks for the input.

have you tried re-installing thunderbird? Try uninstalling, don't forget to backup your data first. Then delete all the left over files manually and re-install the latest version. Restore back the data.

HI Guys,
Thanks for the help. I received a notification from TB on a "webmail" addon update. When this update was done and the problem solved. Appreciate your effort.