It could be lots of times, after you posted your comments or wrote a mail and so on.
In this case, a tool provided by FireFox (FF) could be very useful: Dictionary
To add it to your FireFox, you only have to click on the world "install" of chosen dictionary and let FireFox install it, then you need to restart FireFox.
The dictionary checks the spelling of the words and when they are wrong written it suggests possible solutions (right click on the word and it will appear some possible corrections , look at the picture below, I wrote "word" badly)
From now on your spelling mistakes will be fixed for ever :)
Another utility that can help us, but has nothing to do with the spelling check, is Backwards, the developers state the following features:
*Translate an English word into French, German, Spanish, Russian, Italian, Japanese, Korean, Traditional or Simplified Chinese when mouse cursor hovers over it.
*With one click, remember all context of the word, such as URL of the page and the paragraph which the word belongs to.
*Get Pronunciation of the word with a flash.
*Write a note for the word.
*Search more explanation of the word with one click.
*Review all words and context collected on a build-in page.
I add that the translation function is a bit shallow but useful to get, just in time, the gist of the unknown word and it is easier to use, indeed you need only to place the pointer near the world that you desire to translate.
At the first use, you have to point out which language to put the English words in.
- Go to Tool (FF' s tool bar)->Add-ons.
- You'll find Backwards-> click on "option" button.
- Choose the language in "Dictionary".
If it seems not to work you need to enable it, you should find a little grey cross in the right low corner of FF (take a look at the snapshot below), double click on it, the cross turn into green from that point on Backward is enabled but sometime it could disable on its own and you need to click on the cross again, instead by turning the cross from green into grey you can disable it when, for example, it's annoying you .
Backwords has a helpful feature that allows the user to make a note of the meaning of the word and check it in the future, you have just to click on, respectively, the red cross and the "B" letter that appear when you place the pointer near the word.