Oops, I should have included that it's always the latest version of ColorfulTabs, Windows 8 Pro (64-bit) and Firefox 21.0 (upgraded to 22.0 yesterday). My apologies, Shivanand!
Sorry that it has taken me a few days to respond, but I had think hard about things in order to better understand the somewhat unusual circumstances.
I usually have at least four or five Firefox windows open, with anything from a few tabs to multiple rows of tabs in a window. Every now and then Firefox crashes, and I now see that when I restart Firefox for some reason there has been one 'Thankyou For Installing ColorfulTabs' tab present as well as the Sessions Manager window.
When I select the crashed session [from thelist of all previous sessions] this will cause all of the crashed Firefox separate windows to be re-opened, and it seems to be that the 'Thankyou For Installing ColorfulTabs' tab was getting replicated into EACH of the re-opened windows.
So this time I closed that 'Thankyou For Installing ColorfulTabs' tab BEFORE clicking on the Open button in the session manager, and this seems to have resolved my issue. (I will let you know here if this still hasn't resolved it.)