Sync allows you to see and share data on devices. I can see the open tabs on my phone from my computer and vise versa. I can also send tabs to 1 or more devices on my account. Extensions... not all extensions are made for the phones. https://support.mozilla.org/en-US/kb/sync https://support.mozilla.org/en-US/kb/sync-custom-preferences https://support.mozilla.org/en-US/kb/view-synced-tabs-other-devices https://www.mozilla.org/en-US/firefox/features/sync/ https://hacks.mozilla.org/2018/11/firefox-sync-privacy/