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I've followed the FAQ but it doesn't spell check anything and all words come up underlined. Yes, I have selected English (GB) in the spell check settings; yes, I have uninstalled any other dictionaries (I even removed the Firefox GB Language pack as well); I've tried uninstalling and reinstalling several times with no change.

I've had to go back to the standard GB Language pack with a (bizarrely) US dictionary - at least the majority of words are checked now :-(

EDIT:
Can't reply to the comment below directly but I've tried manually installing the addin as per the instructions but there is no difference.

EDIT 2:
Yes, as mentioned in the original post, I have selected the Dictionary/Language for Spellchecking in an editable field - This is when it switches from just highlighting non-US words to highlighting every word so I know it's made the switch (It just doesn't accept any words as spelled correctly).

Questa recensione riguarda una versione precedente (2.59) del componente aggiuntivo. 

Hello Zog,

Could you try the steps explained in:
http://marcoagpinto.cidadevirtual.pt/en_installing.html
?

Please tell me if this solves your issue.

Thanks.

EDIT: Make sure you have installed on Firefox if it is to be used there. Then, right-click in an editable text area just like this one when you writing in it, and select: "Languages" -> "English (United Kingdom)"