This is triggered on launch with an UBSan build. To enable this check add the following to your mozconfig:

ac_add_options --enable-address-sanitizer
ac_add_options --enable-undefined-sanitizer="null"
ac_add_options --disable-jemalloc

Found with m-c 20200209-be2a7d1a4d0d

netwerk/base/SSLTokensCache.cpp:70:19: runtime error: member call on null pointer of type 'mozilla::net::nsIOService'
    #0 0x7fc3ef01a630 in mozilla::net::SSLTokensCache::Init() netwerk/base/SSLTokensCache.cpp:70:19
    #1 0x7fc3ef06781d in mozilla::net::nsIOService::Init() netwerk/base/nsIOService.cpp:257:3
    #2 0x7fc3ef06a355 in mozilla::net::nsIOService::GetInstance() netwerk/base/nsIOService.cpp:410:9
    #3 0x7fc3eed4d9a2 in mozilla::xpcom::CreateInstanceImpl(mozilla::xpcom::ModuleID, nsISupports*, nsID const&, void**) objdir-ff-ubsan/xpcom/components/StaticComponents.cpp:11012:48
    #4 0x7fc3eedab917 in nsComponentManagerImpl::GetServiceLocked((anonymous namespace)::MutexLock&, (anonymous namespace)::EntryWrapper&, nsID const&, void**) xpcom/components/nsComponentManager.cpp:1372:17
    #5 0x7fc3eeda5434 in nsComponentManagerImpl::GetServiceByContractID(char const*, nsID const&, void**) xpcom/components/nsComponentManager.cpp:1559:10
    #6 0x7fc3f1ca6642 in nsScriptSecurityManager::Init() caps/nsScriptSecurityManager.cpp:1380:17
    #7 0x7fc3f1ca6afb in nsScriptSecurityManager::InitStatics() caps/nsScriptSecurityManager.cpp:1441:28
    #8 0x7fc3f15e5aaa in nsXPConnect::InitStatics() js/xpconnect/src/nsXPConnect.cpp:146:3
    #9 0x7fc3f156a9e8 in xpcModuleCtor() js/xpconnect/src/XPCModule.cpp:11:3
    #10 0x7fc3f83a3f58 in nsLayoutModuleInitialize() layout/build/nsLayoutModule.cpp:108:7
    #11 0x7fc3eeda5f9d in nsComponentManagerImpl::Init() xpcom/components/nsComponentManager.cpp:482:5
    #12 0x7fc3eee8ca70 in NS_InitXPCOM xpcom/build/XPCOMInit.cpp:447:51
    #13 0x7fc3fb72c8c2 in ScopedXPCOMStartup::Initialize(bool) toolkit/xre/nsAppRunner.cpp:1264:8
    #14 0x7fc3fb73a51f in XREMain::XRE_main(int, char**, mozilla::BootstrapConfig const&) toolkit/xre/nsAppRunner.cpp:4697:22
    #15 0x7fc3fb73ac73 in XRE_main(int, char**, mozilla::BootstrapConfig const&) toolkit/xre/nsAppRunner.cpp:4752:21
    #16 0x55bdc26ddda2 in do_main(int, char**, char**) browser/app/nsBrowserApp.cpp:217:22
    #17 0x55bdc26dd4fb in main browser/app/nsBrowserApp.cpp:331:16

A Pernosco session is available here: https://pernos.co/debug/F19Q4SjM2b8tLASWXqbMIw/index.html

Kershaw, please take a look?

https://hg.mozilla.org/mozilla-central/rev/bd3c32f19d2c