Firefox has a handy feature which can check your spelling as you type in forms. To enable this go to Tools > Options > Advanced > Browsing and check 'Check spelling as I type'. If this still isn't working you may need to install your dictionary (even if your dictionary is listed under Tools > Options > Content > Languages). To do this, go to https://addons.mozilla.org/en-US/firefox/browse/type:3 and download your language, install and restart Firefox. You should now have many squiggly red lines in your forms as you type (ok, maybe that's just me :o) ).