The following three paragraphs are editable with contenteditable and also have the attribute spellcheck. You can activate the editing process by clicking on the text. Firefox will then mark the editable area with a dashed border. The attribute spellcheck will be displayed as a check box in the context menu. Click the check box to select it, and off you go with the spell check.
But that is not all! If you click again with the right mouse button, the context menu also displays the available selection of languages. Additionally, suggestions for correcting unknown or misspelled words are offered in the context menu, and you have the option of saving the relevant word to a customized dictionary.
Are you asking yourself where this private dictionary is located? In the user's profile directory, of course. The filename is persdict.dat and has the format text. Unfortunately, customized entries are not yet suggested during spell checking.