This article applies to the deprecated Zotero for Firefox (pre-Zotero 5.0) plugin. It no longer applies to the current versions of Zotero.
Disable all other Firefox extensions, beginning with those that are known to cause problems with Zotero.
If disabling all extensions allows you to open Zotero, re-enable your extensions one-by-one, restarting Firefox each time, until you find the conflicting extension. If the conflicting extension isn't listed on the Known Issues page, report it in the forums. You should also contact the author of the other extension, who will likely need to fix the problem.