French spell checking doesn't work properly:
Indeed every word preceded by an apostrophe is considered as mispelled. There was discussion on that on bugzilla.
https://bugzilla.mozilla.org/show_bug.cgi?id=415254
You can also check on
http://www.dicollecte.org/home.php?prj=fr
This is a dictionary based on hunspell used on :
OpenOffice.org / LibreOffice
Firefox
Chrome / Chromium
Opera
InDesign CS
Linux
Thanks for your attention.