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)
This is the basis of our component technology; this covers the mozilla/xpcom source directory and includes the "repository". Unlikely a tester would be able to tell there was an XPCOM problem specifically.
There are precious few tests for nsITimer currently, and in particular using timers off the main thread is very poorly tested. We should have a test harness which does stress-testing of timers and can be run under ASAN/TSAN.