Spell Check Tweak
Allows the spell checker to check both single and multi-line text area fields

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Tweak Type:	General
Fx 2:	YES
Fx 3:	YES
Fx 3.5:	YES

The new inline spell checker introduced in Firefox 2 is set by default only to check multi-line text areas. What this means is if the text area is a single line (example on Delphi Forums, message and poll titles are a single line text area) you must manually select Spell check this field from the context menu. This can be fixed with a simple modification to about:config.

Click the about:config button (if about:config extension is installed) or open a new tab and type about:config in the address bar and press enter

In the filter field type layout.spellcheckDefault

Double-click on the entry and in the pop-box enter 2

Click OK

Source