This is useful for users who dual-boot their machines. However, it may cause a few problems, especially with extensions. Security Firefox is free-libre software, and thus in particular its source code is visible to everyone. This allows anyone to review the code for security vulnerabilities. It also allowed the U.S. Department of Homeland Security to give funding for the automated tool Coverity to be run against Firefox code. Additionally, Mozilla has a security bug bounty system - anyone who reports a valid critical security bug receives a $3000 (US) cash reward for each report and a Mozilla T-shirt. With effect from December 15, 2010, Mozilla added Web Applications to its Security Bug Bounty Program. Tabbed browsing Firefox supports tabbed browsing, which allows users to open several pages in one window. This feature was carried over from the Mozilla Application Suite, which in turn had borrowed the feature from the popular MultiZilla extension for Mozilla. Firefox also permits the "homepage" to be a list of URLs delimited with vertical bars (|), which are automatically opened in separate tabs, rather than a single page. Firefox 2 supports more tabbed browsing features, including a "tab overflow" solution that keeps the user's tabs easily accessible when they would otherwise become illegible, a "session store" which lets the user keep the opened tabs across the restarts, and an "undo close tab" feature. The tab browsing feature allows users to open multiple tabs or pages on one window. This is convenient for users who enjoy browsing from one window and is also advantageous in ensuring ease of browsing. The tabs are easily made accessible and users can close tabs that are not in use for better usability. Pop-up blocking Firefox also includes integrated customizable pop-up blocking. Firefox was given this feature early in beta development, and it was a major comparative selling point of the browser until Internet Explorer gained the capability in the Windows XP SP2 release of August 25, 2004. Firefox's pop-up blocking can be turned off entirely to allow pop-ups from all sites. Firefox's pop-up blocking can be inconvenient at times — it prevents JavaScript-based links from opening a new window while a page is loading unless the site is added to a "safe list" found in the options menu. In many cases, it is possible to view the pop-up's URL by clicking the dialog that appears when one is blocked. This makes