Yes, you can use the Firefox built-in spell checker from within Zotero. Misspelled words are underlined in red; right-clicking (or ctrl-clicking on Mac OS X) will open a menu with a list of suggested spellings; if you highlight one of these, Firefox will substitute it for the misspelled word. To spell check in languages other than English, go to the Firefox Dictionaries Add-ons page, where you can browse for a dictionary in the language of your choice. After installing the dictionary and restarting your browser, right click (ctrl-click on Mac OS X) from within a Zotero note, highlight “Languages” from the pull-down menu, and select your newly added dictionary.