Printing in a debugger

dump frame tree, dump content tree, dump layer tree, dump displaylist framedump, frame dump

p nsIContent->List(stdout, 0)
p $9->DumpFrameTree() # needs --enable-debug or --enable-dump-painting
p $1->Manager()->Dump()
p PrintDisplayListToStdout(aBuilder, list)
p sDumpPaintList = 1

layout.display-list.dump-content
MOZ_DUMP_PAINT_LIST=1 mach run -p debug

with invalidation:

nglayout.debug.invalidation
MOZ_DUMP_INVALIDATION=1 MOZ_DUMP_PAINT_LIST=1 mach run -p debug
layers.dump-host-layers

print clip

nsCString s = aData->mClip.ToString();