How to force Firefox to sync my bookmark just now?

Firefox → Sync Now, or if you have the menu bar enabled, Tools → Sync Now.

As far as I know, nothing has changed much in the latest versions of Firefox except the user interface :
One sets up the sync in menu "Tools → Options → Sync", then uses "Tools → Sync Now".

If you do not see the menu, right-click an empty space in the above panels and click on "Menu Bar".

@Peltier:

If the problem is with the new panel that is opened by the Firefox button missing the "Sync Now" icon, click the Customize button in that panel, drag the "Sync" button on the left into the panel, then close the customize tab.

You can now click on "Firefox → Sync" to sync now.

Mouse users: Strike an [Alt] key and the menu appears (Firefox 29) then > Tools > Sync Now

Keyboard users: Alt+T > S