Solved Firefox, Synth or not and LTO

FYI, thunderbird failing with LTO is already reported.
And I can confirm this on an independent installation. The build ends with:
ld: error: undefined hidden symbol: JS::CleanupQueuedFinalizationRegistry(JSContext*, JS::Handle<JSObject*>)
>>> referenced by Unified_cpp_xpcom_base0.cpp
>>>               lto.tmp:(mozilla::CleanupFinalizationRegistriesRunnable::Run())

ld: error: too many errors emitted, stopping now (use -error-limit=0 to see all errors)
clang++: error: linker command failed with exit code 1 (use -v to see invocation)

Many undefined symbols before that...