fix search button in firefox

Before: 
![ff_search_1](https://f.cloud.github.com/assets/202576/1872079/5c71779e-78a0-11e3-9a20-a132446ff7bf.png)

After:
![ff_search_2](https://f.cloud.github.com/assets/202576/1872096/8d0ce280-78a0-11e3-92a1-252e8b37c195.png)

tested in chromium, firefox, IE