How to enable Firefox Spell checking?

How to enable Firefox Spell checking?

I can't access the firefox context menu in Zoho Writer's main text area since it is replaced by the Zoho Writer's own menue.

Therefore, I am not able to tick the "Spell check this textarea" menue item, nor to change the firefox language dictionaries. I am aware of the Zoho spell check feature, but I'd rather use the firefox addon.

Is ist possible?
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markus