First things first, have you installed the firefox extension successfully? To check this go to tools -> add-ons and make sure Zotero is listed there. Also check that you have none of the known incompatible extensions installed.
There is a knowledge base article with more information on other things to try here. You could also try switching off any custom themes, as some themes have been known to cause problems with Zotero.
There is a knowledge base article with more information on other things to try here. You could also try switching off any custom themes, as some themes have been known to cause problems with Zotero.