As i see there are only rating based browser protection. There are no binary code modification based protection in Avast antivirus.
There is more than just rating based browser protection - there is also phishing protection.
Not to mention that avast has the Web Shield that is the real-time scanning of web content for malware.
I've heard about sandbox in Avast. It seems it is all we have as protection against unknown browser vulnerability.
The avast sandbox is different again, you can elect to run your browser in the sandbox (isolated), but it would also be scanned. The full sandbox feature is only available in the paid products not the free version.
The file system shield can run suspect files in a sandbox environment (DeepScreen), which is available in the free version.
When Avast detects suspicious behavior in a file or program it is automatically opened or run in the sandbox to prevent any potential damage to your system.
You can also use Hardened mode:
Enable Hardened mode - Recommended for novice users who are not aware of risks when opening unknown files. Set on moderate, no files with poor reputation are allowed to run. Set on aggressive, only whitelisted files can be opened.