My guess:
You're getting a prompt to confirm which add-ons you installed. There used to be a problem with other programs installing add-ons without your permission, and as a result many people didn't know they had those add-ons installed. Sometimes those add-ons caused Firefox to be slow or unstable. However, there are some cases where people are aware of those add-ons being installed by third-parties, and actually want them.

So in a Firefox update a couple of months ago, a one-time confirmation prompt was added to make sure you are aware of all add-ons that are already installed, and let you choose which ones you want to keep.

Here's a blog post about the feature: http://blog.mozilla.com/addons/2011/08/11/strengthening-user-control-of-add-ons/

Is that what you're seeing?

Also, please also try deleting the files named extensions.* in the Firefox Profile Folder after exiting Firefox. You can also open the profile folder through Firefox Help (Alt + T) > Troubleshooting Information > Open Containing Folder and exit Firefox. After restarting Firefox may ask you again, if so choose Allow and Continue in every tab but click Restart only in the last tab.

If the problem persists you can try creating new profile and later copy needed files like places.sqlite which holds the bookmarks and history, key3.db + signons.sqlite which stores the passwords etc. from the old profile and also install the add-ons. Firefox Profile Files

Mr.Vivek,

how do i create a profile... can you explain briefly..

thanks

The Profile Manager - Create, remove or switch Firefox profiles article will tell you how to create profile.

The Recover important data from an old profile article will tell you how to copy your important data to the new profile.

chris thanks to give the solution.. can you explain how do i change that? whether i need to reinstall the firefox or any possibility to change?

You don't have to reinstall. With profiles you get to create different settings, bookmarks, history etc. with a single installation of Firefox which would otherwise have required the use of Firefox in multiple OS user accounts and switching between the user accounts. Type firefox -p in the Windows Start menu Run box to start the profile manager and Create Profile... and switch between the profiles. If Firefox is running type firefox -p -no-remote instead. You'll find the full details and helpful info in the links posted by Chris Ilias and me above. Please also go through the links inside those pages.

Firefox Commands