Context menu (e.g. right click for Windows and Linux / Control click for Macintosh) and select "Check Spelling".

Ctrl-Shift-F7 for Windows and Linux / Cmd-Shift-F7 for Macintosh.

Toolbar button in Firefox select View - Toolbars - Customize and then drag and drop the SpellBound toolbar button onto a toolbar.

The Spellchecker is only available when a text box has focus.