What instructions, you have to give link where your reading those instructions.

Assuming that in Options => General, under 'Language' you have unchecked 'Check your spelling as you type' (?)

Would you try this please :

Type in the address bar about:config and press Enter (promise to be careful, if asked)

Type in the search bar :

layout.spellcheckDefault

and make sure that its value is set to 0 (= zero)

(If you have to change the value, then close and restart Firefox afterwards)

WestEnd said

What instructions, you have to give link where your reading those instructions.

Disabling automatic spell-checking

By default, Firefox is configured to automatically check spelling. You can turn it off in the Options window:

   Click the menu button Fx57Menu and choose Options.
   In the General panel, go to the Language section.
   Uncheck Check your spelling as you type.

   spellcheck 57

   Close the about:preferences page. Any changes you've made will automatically be saved.