After using Firefox for years, I literally just found out how to correct the misspelled words.
On a Mac, hold down the CONTROL key and click on the underlined word. Up will pop a menu with suggested corrections.
It works! No more opening a new tab and typing in "spell ardvark" and then copying the Google Search result, going back to the previous tab and pasting it in. All done without leaving the page. I'd give it five stars but the documentation of how to correct misspelled words was not east to find.