Firefox automatically checks the spelling of words that you enter in text boxes containing more than one line. As soon as you finish typing a word, it is checked against the words in the installed dictionary. If the word is not found in the dictionary, it will be underlined in red, as in the following example:

Note:

• Automatic spell-checking is only turned on for text boxes containing more than one line, where you can enter as much text as you want.
• Not all locales come with a dictionary installed due to licensing reasons.

Disabling automatic spell-checking 

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

1. At the top of the Firefox window, click on the Firefox button (Tools menu in Windows XP) and then click Options
2. Select the Advanced panel.
3. Click the General tab.
4. Uncheck Check my spelling as I type.
5. Click OK to close the Options window

If none of the suggested words is appropriate, you must manually edit it. If you are sure the word is spelled correctly, you can add it to the dictionary. To add a word to the dictionary,right-click on it, and select Add to Dictionary.

Adding dictionaries 

You can install other dictionaries as necessary.

1. Right-click in any text box containing more than one line.
2. Select Languages.
3. Select Add dictionaries…. 

4. The Mozilla Add-ons Dictionaries page is displayed. To add a dictionary to your Firefox installation, click the “Install Dictionary” link next to the dictionary you want to add. The Software Installation window appears. When it is displayed, click Install Now, which should open the Add-ons window, where you’ll see the dictionary being installed.

5. To enable the dictionary, click the Restart Now button, in the Add-ons manager.

Switching dictionaries

After the dictionary is installed, you can switch to it, as needed. To switch to a different installed dictionary, right-click in a multi-line field, select Languages, then select the dictionary you want to use.

Removing a word you have accidentally added

1. At the top of the Firefox window, click on the Firefox button, go over to the Helpmenu (on Windows XP, click on the Help menu) and select Troubleshooting Information. The Troubleshooting Information tab will open.
2. Under the Application Basics section, click on Show Folder. A window with your profile files will open.
   Note: If you are unable to open or use Fire​fox, follow the instructions in Finding your profile without opening Firefox.

   Open your profile folder:At the top of the Firefox window, click on the Firefox button (File menu in Windows XP) and then click Exit

3. At the top of the Firefox window, click on the Firefox button (File menu in Windows XP) and then click Exit
4. Open the file persdict.dat in a text editor (such as Wordpad).
5. In the text editor each word you’ve added is on a separate line. Remove the line, containing the word you wish to remove.
6. Click the File menu, and select Save, to save your work.