(Both www/firefox and www/firefox68)
There's an interesting story to this one. Multiprocess mode never worked properly on NetBSD. When we finally implemented the necessary goo (processes-shared semaphores) it was leaky and would eventually run out of file descriptors and crash. Unfortunately, as of the most recent release, they broke non-multiprocess Firefox, so an urgent fix was required. We couldn't keep it disabled by default.
maya and thorpej eventually discovered that the problem was Firefox attempting to destroy semaphores in processes that hadn't created them, which would fail and leak. But apparently, macOS has the same restriction! I was able to find the code that avoids cross-process semaphores on macOS, and we unconditionally enabled it on NetBSD.
Now Firefox on NetBSD is using an oddball rendering mode normally only used on macOS - but it's fast, stable, and multiprocess. You get all the added robustness you normally would.