Starting it under gdb shows that it crashes due to infinite recursion copying a string:

(gdb) bt
#0  0x00007fffe443ab5d in mozilla::detail::nsTStringRepr<char>::nsTStringRepr(char*, unsigned int, mozilla::detail::StringDataFlags, mozilla::detail::StringClassFlags) (this=<optimized out>, aData=0x7fffea2099c4 <gNullChar> "", aLength=0, aDataFlags=mozilla::detail::StringDataFlags::TERMINATED, aClassFlags=mozilla::detail::StringClassFlags::NULL_TERMINATED) at xpcom/string/nsTStringRepr.h:322
#1  0x00007fffe443ab5d in nsTSubstring<char>::nsTSubstring(mozilla::detail::StringClassFlags) (this=<optimized out>, aClassFlags=mozilla::detail::StringClassFlags::NULL_TERMINATED) at xpcom/string/nsTSubstring.h:1142
#2  0x00007fffe443ab5d in nsTString<char>::nsTString(nsTSubstringTuple<char> const&) (this=<optimized out>, aTuple=...) at xpcom/string/nsTString.h:95
#3  0x00007fffe443ab5d in nsTSubstring<char>::Assign(nsTSubstringTuple<char> const&, std::nothrow_t const&) (this=0x7fffdf6bf040, aTuple=..., aFallible=...) at xpcom/string/nsTSubstring.cpp:556
#4  0x00007fffe443ab82 in nsTSubstring<char>::Assign(nsTSubstringTuple<char> const&) (this=0x7fffea2099c4 <gNullChar>, aTuple=...) at xpcom/string/nsTSubstring.cpp:546
#5  0x00007fffe443ab82 in nsTString<char>::nsTString(nsTSubstringTuple<char> const&) (this=0x7fffea2099c4 <gNullChar>, aTuple=...) at xpcom/string/nsTString.h:96
#6  0x00007fffe443ab82 in nsTSubstring<char>::Assign(nsTSubstringTuple<char> const&, std::nothrow_t const&) (this=0x7fffdf6bf090, aTuple=..., aFallible=...) at xpcom/string/nsTSubstring.cpp:556
#7  0x00007fffe443ab82 in nsTSubstring<char>::Assign(nsTSubstringTuple<char> const&) (this=0x7fffea2099c4 <gNullChar>, aTuple=...) at xpcom/string/nsTSubstring.cpp:546
#8  0x00007fffe443ab82 in nsTString<char>::nsTString(nsTSubstringTuple<char> const&) (this=0x7fffea2099c4 <gNullChar>, aTuple=...) at xpcom/string/nsTString.h:96
#9  0x00007fffe443ab82 in nsTSubstring<char>::Assign(nsTSubstringTuple<char> const&, std::nothrow_t const&) (this=0x7fffdf6bf0e0, aTuple=..., aFallible=...) at xpcom/string/nsTSubstring.cpp:556
#10 0x00007fffe443ab82 in nsTSubstring<char>::Assign(nsTSubstringTuple<char> const&) (this=0x7fffea2099c4 <gNullChar>, aTuple=...) at xpcom/string/nsTSubstring.cpp:546
#11 0x00007fffe443ab82 in nsTString<char>::nsTString(nsTSubstringTuple<char> const&) (this=0x7fffea2099c4 <gNullChar>, aTuple=...) at xpcom/string/nsTString.h:96
#12 0x00007fffe443ab82 in nsTSubstring<char>::Assign(nsTSubstringTuple<char> const&, std::nothrow_t const&) (this=0x7fffdf6bf130, aTuple=..., aFallible=...) at xpcom/string/nsTSubstring.cpp:556
#13 0x00007fffe443ab82 in nsTSubstring<char>::Assign(nsTSubstringTuple<char> const&) (this=0x7fffea2099c4 <gNullChar>, aTuple=...) at xpcom/string/nsTSubstring.cpp:546
#14 0x00007fffe443ab82 in nsTString<char>::nsTString(nsTSubstringTuple<char> const&) (this=0x7fffea2099c4 <gNullChar>, aTuple=...) at xpcom/string/nsTString.h:96
#15 0x00007fffe443ab82 in nsTSubstring<char>::Assign(nsTSubstringTuple<char> const&, std::nothrow_t const&) (this=0x7fffdf6bf180, aTuple=..., aFallible=...) at xpcom/string/nsTSubstring.cpp:556
#16 0x00007fffe443ab82 in nsTSubstring<char>::Assign(nsTSubstringTuple<char> const&) (this=0x7fffea2099c4 <gNullChar>, aTuple=...) at xpcom/string/nsTSubstring.cpp:546
#17 0x00007fffe443ab82 in nsTString<char>::nsTString(nsTSubstringTuple<char> const&) (this=0x7fffea2099c4 <gNullChar>, aTuple=...) at xpcom/string/nsTString.h:96
#18 0x00007fffe443ab82 in nsTSubstring<char>::Assign(nsTSubstringTuple<char> const&, std::nothrow_t const&) (this=0x7fffdf6bf1d0, aTuple=..., aFallible=...) at xpcom/string/nsTSubstring.cpp:556
#19 0x00007fffe443ab82 in nsTSubstring<char>::Assign(nsTSubstringTuple<char> const&) (this=0x7fffea2099c4 <gNullChar>, aTuple=...) at xpcom/string/nsTSubstring.cpp:546
#20 0x00007fffe443ab82 in nsTString<char>::nsTString(nsTSubstringTuple<char> const&) (this=0x7fffea2099c4 <gNullChar>, aTuple=...) at xpcom/string/nsTString.h:96
#21 0x00007fffe443ab82 in nsTSubstring<char>::Assign(nsTSubstringTuple<char> const&, std::nothrow_t const&) (this=0x7fffdf6bf220, aTuple=..., aFallible=...) at xpcom/string/nsTSubstring.cpp:556
#22 0x00007fffe443ab82 in nsTSubstring<char>::Assign(nsTSubstringTuple<char> const&) (this=0x7fffea2099c4 <gNullChar>, aTuple=...) at xpcom/string/nsTSubstring.cpp:546