I assume that you do have a dictionary for spell check installed and not a language pack. A webpage can disable the Firefox spell checker via a spellcheck="false" attribute of an input field or textarea or editable element and possibly provide its own spelling checker. You can right-click in the text area or input field and use "Inspect Element" to check in the Inspector if this text area has a <b>spellcheck="false"</b> attribute. You can remove this spellcheck="false" attribute or change it to spellcheck="true" via a double-click or via "Edit As HTML" to make the "Check Spelling" item in the right-click context menu reappear. Firefox will disable "Check Spelling" in the right-click context menu of a text area if the web page disables the spelling checker via a spellcheck="false" attribute of this element. You can right-click and select "Inspect Element" to open the builtin Inspector with this element selected. *https://developer.mozilla.org/en-US/docs/Tools/Page_Inspector/How_to/Examine_and_edit_HTML