How do I enable Firefox check spelling on single line text-boxes forever?

As you know, we can enable the spell checker on single line text boxes in the following way:

We have to enable it every time for checking words.

How can we enable it forever, so it doesn't need to be enabled each time?

Answer

To change how Firefox checks spelling and enable spell-checking on one-line and multi-line text fields, use about:config to set layout.spellcheckDefault to 2.

To edit your configuration, type about:config in the address bar and press Enter.

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