Here is the way to disable add-ons window which appears every time when selenium scripts are run on Custom Firefox Profile.
Close all instances of Firefox browser and delete the following files from the Custom Profile folder, this should reset Extension Manager and disable add-ons pop-up:
extensions.cache
extensions.ini
extensions.rdf
and
compatibility.ini
This Add-ons pop-up will not displayed the next time you run selenium scripts.
Submitted by Irina Burdilo

Thanks a Lot !! It worked like a magic for me….
It works perfectly. I can’t believe I got rid of that pop-up at last. Thanks!.
Irina…thank you soooo much for your awesome solution…it worked perfectly right off the bat!!!
Here’s how to find Custom Profile Folder
Here’s how ti find Custom Profile Folder
http://kb.mozillazine.org/Profile_folder_-_Firefox
thanks for your solution. it worked for me..