1. A computer system including instructions stored on a computer-readable medium and executable by at least one processor to execute a browser application and thereby provide a browser interface, the computer system comprising: the at least one processor; a rendering engine configured to cause the at least one processor to execute, within a first execution environment of the rendering engine, a page script of a page to be rendered within the browser interface, the page script configured to interact with a page model to implement the rendering of the page; an extension manager configured to cause the at least one processor to render, in an extension execution environment, at least a portion of an extension file which is configured to modify a functionality of the browser application in association with the rendering of the page by the rendering engine, including detecting a content script associated with the extension file which, during execution, interacts with the page model; and an environment manager which is configured to cause the at least one processor to create a second execution environment of the rendering engine for execution of the content script file therein, including providing a copy of the page model within the second execution environment of the rendering engine, wherein the content script, during execution within the second execution environment of the rendering engine, interacts with the copy of the page model independently of how the page script, during execution within the first execution environment of the rendering engine, interacts with the page model, and further wherein the extension manager is configured to cause the at least one processor to render the at least a portion of the extension file based on the execution of the content script within the second execution environment of the rendering engine.