I think it depends on what version of Windows you have. Windows 7 underlines as you type but Vista only underlines when you click the ABC spell check button.
I haven't yet checked on Windows 8.1 or XP.
I'm on W8.1. Need to click button.
I can confirm that having made replies today using both quick, and WSYWIG editors, the WYSYWIG requires that the "spell" button be clicked. Also the button acts as a toggle, so requires to double clicked (off then on again) to pick up any subsequent mistakes, and if typing beyond a mistake is made, the red underline continues regardless.
This is contrary to usual forum behaviour elsewhere, and using the full editor does appear to block any third party inline spell checkers. Hopefully, this can be reported to whatever forum software issue tracker there might be.
Have been reading through the answers here, because i have also had the same problem. My normal Firefox add-on dictionary does not work on the forum. Thanks to the previous answers i have now found out i can at least use the one built into the forum software. that is fine, but is rather annoying to have to do the check manually, i have not had to do that for years.
Someone mentioned that maybe the forum spell checker is the reason why my normal Firefox one will not work, not sure if that is the case, because it would seem the forum one is always switched off until such time you click the spell checker button. If it is running all the time, i wish it would chirp up and let me know when i am making a typo.
I guess it comes down to what you are familiar with, spelling mistakes being underlined the second you make them, or having to do it manually, i prefer the first option, but can live with what we have got, so many thanks for pointing me to the spell check button - yeah, the one that's been sitting right in front of my eyes for the last few months, lol.
But if a solution is found to why browser add-on spell checkers do not work, would be good to hear it.