Go to about:config and search for layout.spellcheckDefault. Click the pencil at the end of the line and change the number from 1 to 2. Click the tick which has replaced the pencil.

Do you have a dictionary installed and selected?

You can see which dictionary is selected if you right-click in a text area and open the Languages sub menu. At least one dictionary should be installed and have a check mark to indicate that it is selected.

• open the "Add Dictionaries" link to install a dictionary if you do not have one.

The spelling checker is enabled if [X] "Check Spelling" in the right-click context menu has a checkmark.

You can enable or disable spell checking globally:

• Settings -> General: Language and Appearance: "Check your spelling as you type"
• https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker

You can look here for dictionaries:

• https://addons.mozilla.org/firefox/language-tools/

Note that for spell check you need a dictionary. A language pack provides string translations for the user interface.

• https://support.mozilla.org/en-US/kb/use-firefox-another-language

You can check in about:config whether layout.spellcheckDefault is set to 1 (multi-line) or 2 (single-line).

(edited to fix broken link)

thepillenwerfer,cor-el,

    Thank you for your replies. I did what you suggested thepillenwerfer. I'll see if that solves the problem? Specifically, when I visit Facebook and reply to a message that was sent to me is where the problem exists. When I type a message other than when I am  replying to a message sent in messenger, the spell checker works. The non spell checking only seems to not be working when I reply in a message that was sent to messenger, not Facebook in general. Sort of peculiar how in one phase of Facebook spell check works but not in messenger?  

cor-el, when I try and access, "https://addons.mozilla.org//language-tools/" that page is unavailable? Can longer be accessed? So, the dictionary issue maybe is still there? Your comment "You can check in about:config whether layout.spellcheckDefault is set to 1 (multi-line) or 2 (single-line)." I did what was suggested and changed the 1 to a 2 in the layout.spellcheckDefault. What will that do? Will that enable spell check in Facebook Messenger? Can either of you two please explain..Thank you both.

Sorry about the broken link, should be:

• https://addons.mozilla.org/firefox/language-tools/

As it comes Firefox only checks the spelling when you type into multi-line boxes. Changing that setting to 2 tells it to also check single-line ones which will hopefully do what you want. I'm not a Facebook user but it has solved the problem on other sites that I use.

If your spell check works elsewhere, as your first post suggests, you must have a suitable dictionary installed already.

thepillenwerfer, thank you for your reply, it is appreciated. I tried your suggestion going to #2 as you said. That didn't make any difference when I respond to a message in Facebook Messenger. Still no spell checker there. Strange though, as I said it works else where in Facebook? Being I do have spell checker every where else, the issue about a dictionary is moot. Also sort of strange how Facebook itself can't fix the problem of no spell checker when a user accesses the messenger feature? Again, thanks for your help, thepillenwerfer.

The website 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" to check in the Inspector if this text area has a spellcheck="false" 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" to open the built-in Inspector with this element selected.

• https://firefox-source-docs.mozilla.org/devtools-user/page_inspector/how_to/open_the_inspector/
• https://firefox-source-docs.mozilla.org/devtools-user/page_inspector/how_to/examine_and_edit_html/

cor-el,

  Thank you for your reply. When I accessed inspect that opened a window. I then accessed   properties and then I scroll down near the bottom of the page I do see spell check=false. Double right clicking did nothing, no response. I also couldn't find "Edit As HTML" as you suggested anywhere in that properties category. Being right clicking did nothing, maybe as you suggest, "Edit A HTML" may do something? But, I can't find a way to access it? Anyway you suggest accessing Edit A HTML so mayne that might change spell check=false to true? Any suggestions, cor-el? Thank you.

You can try to use a bookmarklet to toggle this attribute. You can create a new bookmark and paste the JavaScript code in its URL field where you would normally enter the address of the website.

For a plain text area like used in this forum you can use this JavaScript code. It works on the currently selected/active element, so you need to click in this text field beforehand to set focus to it.

javascript:void(document.activeElement.setAttribute('spellcheck','true'));

For a rich-text editor you can try this code.

javascript:void(document.activeElement.contentDocument.querySelector('*[spellcheck]').setAttribute('spellcheck','true'));

cor-el,

    Thank you for you reply. So, when its all said and done, will changing spell check: false to spell check: true solve the Facebook Messenger problem? Again, I have no problem with spell check ANY where else, just trying to reply to a Facebook message in Facebook Messenger specifically. Strangely, spell check works elsewhere in Facebook. Sort of strange how spell check works in one portion of Facebook and not another??? I do appreciate your help. Thank you, cor-el.

Using the Inspector, I noticed that the only spellcheck attribute on the main Facebook.com page refers to anything typed in the Search Facebook bar. If you are chatting with contacts using the Messenger sidebar, there will be no spell check in your messages.

To get spellcheck in your messages, click the Messenger icon & then "See all in Messenger" at the bottom of your list of chats. (or go to http://www.facebook.com/messages) Once you're in Messenger, the spellcheck attribute should be set to on by default. That was my experience just now.

Hello bodybreak, thank you for your reply. You hit the nail on the head! I appreciate your help and assistance in solving my problem, thank you, bodybreak!