Now that Firefox 2 has built in spell check, it still has one issue, it doesn’t automatically spell check all fields by default. The normal behavior is to only spell check multi-line text fields. If you want to have it always spell check all text fields you need to follow these three simple steps.
- Go to about:config in your URL bar
- Search for spellcheckDefault
- Change the value of spellcheckDefault to 2
That’s it. Now Firefox will spell check all input boxes automatically. 
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Very very useful! Thanks!