Enhance or complement a website: Use an add-on to deliver additional in-browser features or information from your website. Allow users to collect details from the pages they visit, to enhance the service you offer.
Let users show their personality: Browser extensions can manipulate the content of pages browsed by a user. Help the user add a favorite logo or picture as the background to every page they visit. Extensions also give you the ability to update the look of the Firefox UI. (Update the Firefox UI using standalone theme add-ons too).
Add or remove content from web pages: You might want to help users block intrusive ads from web pages, provide access to a travel guide whenever a country or city is mentioned in a web page, or reformat page content to offer a consistent reading experience. With the ability to access and update both a page’s HTML and CSS, you can help users see the web the way they want to.
Add tools and new browsing features: Add new features to a taskboard, or generate QR code images from URLs, hyperlinks, or page text. With flexible UI options and the power of the WebExtensions APIs you can easily add new features to a browser. And, you can enhance almost any website’s features or functionality, it doesn't have to be your website.
Games: Offer traditional computer games, with off-line play features, but also explore new game possibilities, for example, by incorporating gameplay into everyday browsing.
Add development tools: You may provide web development tools as your business or have developed a useful technique or approach to web development that you want to share. Either way, you can enhance the built-in Firefox developer tools by adding a new tab to the developer toolbar.
Extensions for Firefox are built using the WebExtensions APIs, a cross-browser system for developing extensions. To a large extent, the API is compatible with the extension API supported by Google Chrome and Opera. Extensions written for these browsers will in most cases run in Firefox or Microsoft Edge with just a few changes. The API is also fully compatible with multiprocess Firefox.