Shared components used by Firefox and other Mozilla software, including handling of Web content; Gecko, HTML, CSS, layout, DOM, scripts, images, networking, etc. Issues with web page layout probably go here, while Firefox user interface issues belong in the Firefox product. (More info)
Easiest steps to reproduce (there are other ways):
1. Open several tabs.
2. Move the keyboard focus to a tab.
3. Open a menu.
4. Use arrow keys to navigate the menu.
Expected result: Tab does not switch.
Actual result: Tab switches.