Somewhere in the forum... I have seen these troubles/issues. I had it too and it was solved. Some of the answers are like this.
Delete your config file:
1. Open Windows Explorer and navigate to: C:\program files\steam\steamapps\common\call of duty modern warfare 3\players2
2. Locate and delete config_mp.cfg
Next, Verify the File Integrity of your game within Steam:
File Integrity Check:
1. Load Steam
2. From the Library section, right-click on the game and select Properties from the menu.
3. Select the Local files tab and click the Verify integrity of game cache... button.
4. Steam will verify the game's files - this process may take several minutes.
5. Once the process is completed, the Check Window will automatically exit.
Check the source: http://community.callofduty.com/thread/200426962