When your mouse pointer is over a movable area, the mouse pointer should turn into a double-arrow. If you notice that after dragging and releasing the mouse, the mouse pointer stays a double arrow, try positioning the mouse pointer where you want the new boundary and click again. The mouse pointer should change back to a standard pointer.

Does that work?

(This suggestion is based on behavior I've seen with some Infragistics ASP.Net controls in IE8.)

No. The mouse pointer does not change to normal state after clicking on the desired position. It continues to display "double arrow" and the column size continues to drift until I close the page and reopen the same.

Sounds like a bug related to drag-and-drop, but hard to identify the cause without getting access to the software or other software that has the same problem.

I am posting a screenshot here if it helps see the problem. In this screen I dragged the Work Item Type column and you can see that the black line still continues though i have moved out of that area. I highlighted Substatus field to indicate that I have moved out from the grid and clicked elsewhere. Hope this helps!

Have you tried using the Firefox SafeMode, to see if that problem disappears?

https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

Yes. I tried in safemode. The problem still persists.