Firefox debug version has a lot of assertion warnings.

I felt confused that they are not got fixed even in latest version 3

A way to disable the warning is here:

When making a debug build of Firefox from the CVS trunk you get lots of warning boxes appearing when it runs. These warnings are about failed assert checks and can usually be ignored if you’re not working on that area directly.

To turn off these warnings you can set an environment variable on Windows as follows:

SET XPCOM_DEBUG_BREAK=warn
firefox -Profilemanager

On Linux, something like:

XPCOM_DEBUG_BREAK=warn ./firefox -ProfileManager