You can have firefox automatically check your spelling as you type. In Preferences, click on the "Advanced" tab, and then on the "General" tab, and you'll see a box you can check that says, "Check my spelling as I type." Maybe you already do that.

Do you mean the feature of using CTRL+click to get spelling suggestions on a word firefox has identified as possibly misspelled? 'Cause that works for me (OSX Snow Leopard, and on all my earlier versions of OSX too). Even if I CTRL-click on a word that's not identified as possibly misspelled, on the menu that pops up there is a choice that says "Check Spelling." If it's identified as possibly misspelled, suggestions + "Add to dictionary" and "Ignore" also appear on that menu. Are you not seeing that menu?

No I was not seeing that menu indeed. I was able to fix it though by disabling most of my add-ons. One of them must be incompatible and kill the whole spell checker part of the menu. With only dictionary add ons enabled, that menu appears as it should indeed. http://fplanque.com/