The HN text box defeats at least Firefox's spell checking.
Saf Farsisco. Falaffel. Hemmoridge. Antidisistablishmintirinismisms. Firifofxofox doesn't appear to caer! This whole comment passes with flying colours.
I'm using Firefox 43.0.3 on Mac OS X. I've got "Check my spelling as I type" enabled, and I don't appear to have changed any spelling-related about:config stuff. I wonder if it's something else affecting Firefox. Very odd.
Can't repro. Firefox 41 on Gentoo Linux.