functionality is varied; such as those enabling mouse gestures, those that block advertisements, and those that enhance tabbed browsing. Features that the Firefox developers believe will be used by only a small number of its users are not included in Firefox, but instead left to be implemented as extensions. Many Mozilla Suite features, such as IRC chat (ChatZilla) and calendar have been recreated as Firefox extensions. Extensions are also sometimes a testing ground for features that are eventually integrated to the main codebase. For example, MultiZilla was an extension that provided tabbed browsing when Mozilla lacked that feature. While extensions provide a high level of customizability, PC World notes the difficulty a casual user would have in finding and installing extensions as compared to their features being available by default. Most extensions are not created or supported by Mozilla. Malicious extensions have been created. Mozilla provides a repository of extensions that have been reviewed by volunteers and are believed to not contain malware. Since extensions are mostly created by third parties, they do not necessarily go through the same level of testing as official Mozilla products, and they may have bugs or vulnerabilities. Like applications on Android and iOS, Firefox extensions have permission model: for example before installing of extension user must agree that this extension can have access to all webpages, or maybe have permission to manage downloads, or have no special permissions — in such way the extension can be manually activated and interact with current page. From 2019 Firefox, Chromium based browsers (Google Chrome, Edge, Opera, Vivaldi) have the same format of extension: WebExtensions API, this is mean that web extension developed for Google Chrome can be used on Firefox (in most cases), and vice versa. Themes Firefox also supports a variety of themes for changing its appearance. Prior to the release of Firefox 57, themes are simply packages of CSS and image files. From Firefox 57 onwards, themes consist solely of color modifications through the use of CSS. Many themes can be downloaded from the Mozilla Update web site. Language packs Language packs are dictionaries for spell checking of input fields. Plugins Firefox supports plugins based on Netscape Plugin Application Program Interface (NPAPI), i.e. Netscape-style plugins. As a side note, Opera and Internet Explorer 3.0 to 5.0 also support NPAPI. On June 30, 2004, the Mozilla Foundation, in partnership with Adobe, Apple, Macromedia, Opera, and Sun