If you're using Firefox 31 and above, click the button with the 3 horizontal lines on it (not sure what it's called) and click the link towards the bottom that says sync (not the actual button). This will guide you through setting up or logging into a Sync account.

The button is usually on the top-right, and the sync option link is right above where it says customize.

You can't inspect content stored on the Sync server online on a web page. You can only do that on a connected computer by inspecting the related items. I'm not using Sync, so I don't know how synced bookmarks are designated. For tabs from other devices there is a section in the History drop-down list.