Web Defuser от Gear

This extension's goal is to put control over web experience back into the hands of the user above all else. If you've ever been frustrated by some annoying website behavior or missing functionality, this extension will empower you to overcome that.

To achieve that, you can define rules to process any HTTP request as you see fit, including modifying headers, body and more.

Please check out the source code repository (with snippets section containing some example useful rules) and interactive documentation on rules.