There is a compiler warning about thread safety in the XHR String implementation.
01:18:37 41:24.10 In file included from Unified_cpp_dom_xhr0.cpp:29:
01:18:37 41:24.11 /home/jenkins/agent/workspace/foxhound/dom/xhr/XMLHttpRequestString.cpp:50:5: warning: reading variable 'mData' requires holding mutex 'mMutex' [-Wthread-safety-analysis]
01:18:37 41:24.11 50 | mData.Taint().concat(aTaint, aIndex);
01:18:37 41:24.11 | ^
There is a compiler warning about thread safety in the XHR String implementation.