Reporting bugs in Firefox is intimidating to say the least. Will a bug filed at GitHub like https://github.com/mozilla/pdf.js/issues/4761 still reach the right place?
bgirard's profiling work in this thread has been amazing, but I don't think I have the knowledge to interpret the profiler results well enough to file bugs on slow PDFs. A way to post "slow" PDFs to be picked up by pdf.js developers would help.