TST Active Tab in Collapsed Tree is a Firefox addon designed to enhance the functionality of the Tree Style Tab extension. This tool is particularly useful for users who manage numerous tabs by grouping them into trees for different tasks. With this addon, users can quickly access their last active tab in a collapsed tree without the need to expand it, streamlining the workflow and improving productivity. The addon requires Tree Style Tab versionor later and utilizes the Tab Extra Contents API for its features.

Customization options are available through the use of the :part() pseudo element selector, allowing users to modify the appearance of the active tab indicator. For instance, users can change the background color of the active tab to better fit their preferences. Detailed part names can be accessed via the remote debugger for Tree Style Tab, ensuring users can tailor their experience to their liking. This addon is subscription-based and falls under the personalization category.