steal*

So you just have a general concern of this happening or you're wondering about a specific addon?

Once you have entered the Master Password to login to the software security device than any extension or person that has access to your computer can inspect passwords stored in the Password Manager. If you install extensions from the Add-ons website then those have been inspected and can be trusted not to do things like stealing passwords. If you have malware on your computer either as a Firefox extension or installed in the Windows OS then nothing can be guaranteed.

• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• Malwarebytes' Anti-Malware:
  http://www.malwarebytes.org/mbam.php

I talk only about the addons i download from the addons of firefox website https://addons.mozilla.org If they safe , and some one check them before publish ?

ddrhosting said

I talk only about the addons i download from the addons of firefox website https://addons.mozilla.org If they safe , and some one check them before publish ?

Most are checked before they're published. Unchecked ones have warnings on the page.

ddrhosting said

I talk only about the addons i download from the addons of firefox website https://addons.mozilla.org If they safe , and some one check them before publish ?

Almost all checked add-ons on the official Mozilla Firefox add-ons page are safe. Unsafe ones will be caught and removed.

If you download add-ons from third party websites (websites not affiliated with Mozilla), you're taking a risk, because they can be viruses or spyware.