Here is the first ever release of Hindi Spell checker plugin / addon for Firefox. It works well in FF version through 1.5 to 3.x.
Download and install Hindi Spell checker Addon for firefox from here.
To use, type in Hindi in any input box within Firefox and right click anywhere in Input box and then select Hindi Spell Check. Misspelled words will be underlined with red marks. Right click on them and select the suggested, appropriate word. You can add new Hindi word by selecting Add this Word.
Here is some example -
Check spelling on Google Indic Transliteration :
Check spelling in Blogger post editor :
Misspelled Word suggestion:
For detailed instructions to how to use Hindi Spell Checker Firefox plugin, pl see this page (Hindi)
Thanks to G. Karunakar for providing Hindi-Words (Approx. 1 Lakh, some needs proof-reading though.)