What do you mean exactly with spellcheck in toolbar? Do you want to have a shortcut to select language or something like that?

Actually you just have to right-click on a word to get spelling suggestion.

See this - https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker

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 submenu. 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.
• Make sure that [X] "Check Spelling" in the right-click context menu is check-marked.

You can enable or disable spell checking globally:

• Tools > Options > Advanced : General: Browsing: "Check my spelling as I type"

You can look here for dictionaries:

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

• http://kb.mozillazine.org/Dictionaries
• http://kb.mozillazine.org/Spell_checking