Actually, Zotero has full support in Mozilla Firefox. In Chromium browser (and Google Chrome), it has only Zotero Connector. Zotero Connector for Chromium is an extension to work with Zotero Stand Alone (or with the cloud Zotero server). It has no Zotero user interface like the one for Firefox. Here how to install it.
1. Go To The Official Website
Using Chromium (or Google Chrome), go to https://www.zotero.org/download/ and click the Chrome button below the Stand Alone column.
2. Install The Extension
The browser will be redirected to Google Chrome Webstore. Click Add to Chrome button to install Zotero Connector.
3. Finished
While finished, you'll noticing Zotero icon appears in address bar when you visit a supported page.