Other than cited stability concerns and the lack of manpower (per Mozilla, even though they actually still patch it themselves on Windows builds), I didn't find much.
PGO Builds (after patching the Makefile to restore the compile time option) do still work in the tree, up until the linker haults with undefined reference to `gfxShapedWord* gfxFont::GetShapedWord. This was a known issue that occured in GCC6+ and was fixed in m-c 1389436.
Per the bug report:
Applying the patch with minor modifications allows PGO to again build.The reason this happens is that there's a template method declared in gfxFont.h and defined in gfxFont.cpp, and used in both gfxFont.cpp and gfxTextRun.cpp. The split was done 3 years ago, and we've been lucky until now, that the instantiation of the template in gfxFont.cpp was not optimized out... until GCC 6 PGO, which, apparently, happily inlines the function in gfxFont.cpp, such that there's nothing left for gfxTextRun to find.
If this is a feature we would like to provide teir3 support for in the tree I can get an Issue/PR going. Obviously this is not needed for all applications, and is primarily a GNU/Linux optimization. I was just curious if there was any downside to patching our applications with PGO support.