I have this too. With Firefox 27 never had that but this started with Firefox 28. Seems that it is still there with Firefox 29 (I myself did revert back to Firefox 28 like many others, after trying that thing called Firefox 29)

Any ideas how to resolve this? We're planning to go live soon but this could be a showstopper.

Error message: A script on this page may be busy, or it may have stopped responding. You can stop the script now, or you can continue to see if the script will complete.

Script: http://localhost:20778/PSTA_EmployApplication/EmployApp.aspx?_TSM_HiddenField_=ToolkitScriptManager1_HiddenField&_TSM_CombinedScripts_=%3b%3bAjaxControlToolkit%2c+Version%3d4.5.7.123%2c+Culture%3dneutral%2c+PublicKeyToken%3d28f01b0e84b6d53e%3aen-US%3ae669ce41-1aa1-4541-aae9-fa5dc37e70db%3ade1feab2%3af9cec9bc%3ae4bd8421%3af2c8e708%3a720a52bf%3a7311d143%3a589eaa30%3a698129cf%3ae148b24b%3a987bb99b%3aa4b66312%3afcf0e993%3afb9b4c57%3accb96cf9%3aab09e3fe%3a87104b7c:12

Strange, try reinstalling Firefox and see if it fixes your issue.

Certain Firefox problems can be solved by performing a Clean reinstall. This means you remove Firefox program files and then reinstall Firefox. Please follow these steps:

Note: You might want to print these steps or view them in another browser.

1. Download the latest Desktop version of Firefox from http://www.mozilla.org and save the setup file to your computer.
2. After the download finishes, close all Firefox windows (click Exit from the Firefox or File menu).
3. Delete the Firefox installation folder, which is located in one of these locations, by default:
   • Windows:
     • C:\Program Files\Mozilla Firefox
     • C:\Program Files (x86)\Mozilla Firefox
   • Mac: Delete Firefox from the Applications folder.
   • Linux: If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it - see Install Firefox on Linux. If you downloaded and installed the binary package from the Firefox download page, simply remove the folder firefox in your home directory.
4. Now, go ahead and reinstall Firefox:
   1. Double-click the downloaded installation file and go through the steps of the installation wizard.
   2. Once the wizard is finished, choose to directly open Firefox after clicking the Finish button.

More information about reinstalling Firefox can be found here.

WARNING: Do not run Firefox's uninstaller or use a third party remover as part of this process, because that could permanently delete your Firefox data, including but not limited to, extensions, cache, cookies, bookmarks, personal settings and saved passwords. These cannot be recovered unless they have been backed up to an external device!

Please report back to see if this helped you!

Thank you.

Thanks for the advice. I followed all the steps suggested but once reinstalled I tried the app again and got the same error. The app works fine on IE and Chrome, the problem is only with Firefox.

Found the solution! The problem is with the AsyncFileUpload. Solution can be found at http://ajaxcontroltoolkit.codeplex.com/workitem/27489