Firefox will be happy to make use of this. They've always run into LTO limitations, especially with the address space of a 32bit MSVC. Maybe with this in place and Clang's CL.exe emulator frontend, they could use it for Windows builds as well, though I suppose there are some blockers like MSVC having the best win32 codegen and them wanting to leverage that. At the very least this should allow more Linux distros to build with LTO.
Hmm why not gsplit-dwarf to really minimize junk passed to the linker?
I'd guess that a lot of the debug info has to be regenerated during global optimization. Line information, function information and all that often won't make sense afterwards.
I wonder if the old-school technique of #include'ing all .c files in one mega.c file and compiling only that, as done by Doom in the 90s, would still work today.