Firefox spell check requires dictionary

Maria H.'s version of Firefox didn't highlight misspelt words in multi-line fields, although the Check my spelling as I type option is checked. I found that I had a English / United States dictionary installed (probably from a previous installation) while she didn't, so the spell check worked on my PC but not on hers. It seems that the Firefox spell checker only works after you install a dictionary; another one of those things that are obvious in hindsight.