Help! Since 2 days no browser is working properly in my Macbook, no internet!

This is an iMac forum, would you like your post moved to the correct forum?

I also use Avast Mac Security

Shut it off, repair permissions and restart your computer. If this works, uninstall Avast as there are no known OX viruses.

Check out User Tip: Viruses, Trojans, Malware - and other aspects of Internet Security and Mac Malware Guide.

Check out this thread & note Linc Davis’s Nov 18, 2014 9:24 PM response regarding MALWARE.

"Avast" is the worst of the whole wretched lot of commercial "security" products for the Mac. Not only does it fail to protect you from any real danger, it may send personal data (such as web browsing history and the contents of email messages) back to the developer without your knowledge, give false warnings, destabilize and slow down the computer, expose you to network attack, and corrupt the network settings and the permissions of files in your home folder. Removing it may not repair all the damage.

Some versions of the product also inject advertising into web pages. In short, apart from the fine print in the license agreement, Avast is indistinguishable from malware, and is arguably worse than any known malware now in circulation.

Back up all data, then remove Avast according to the developer's instructions. Restart.

If you tried to remove Avast by dragging an application to the Trash, you'll have to reinstall it and then follow the instructions linked above.