After using this for a while, I've had to edit my review and lower my rating from 4 to 3 stars.
Why? I was disappointed to find it didn't work in Yahoo Mail, where I planned to use it most.
Also, it's annoying that its window is anchored to FF's upper left corner rather than being movable. I mean, this isn't 1993.
And as I mentioned before, when you install SpellBound a dialog box appears asking if you want to change your Home page to one where SB's developer apparently has a financial tie-in. That's not spyware, you have a choice—but it is tacky, something you don't expect FF add-ons to do.
But the real deal-breaker for me was that I realized FF's built-in spell-checker DOES let you save words. Duh! How'd I miss that? That's really the only reason I was looking for a third-party checker.
I'm sure this developer meant well, but this add-on seems unnecessary now.