There are plenty of dictionary add-ons for Firefox, but users who spend a lot of time online might be looking for more comprehensive help. If you are, SpellBound is a great option. This add-on comes with American English pre-selected, but it uses the spell-checking power of other Firefox dictionaries so you can make sure your writing is word-perfect in any language.
When you write online, say in a forum or blog, you can access SpellBound by right-clicking on the text. The SpellBound interface opens up, displaying the text, any mistakes and some standard spellchecking options, such as Ignore or Add To Dictionary. You can switch between the languages you have installed, make your own changes or edit your custom dictionary.
Anybody who has ever used a standard offline spellchecker will have no problem using SpellBound. It features all the normal features and is very easy to use. There are a few configuration options - accessed through Tools > Add-ons > SpellBound - that allow you to add new dictionaries or change the font and color of spellchecked words. It's true that there are several spellcheckers for Firefox that do a similar job in a smaller space, but if you spend a lot of time online, the extra functionality of SpellBound will come in very uselful.
SpellBound is a good spellchecker for frequent online writers.