But there are builds of Chromium that support Firefox Sync – Samsung’s Android browser even uses Firefox Sync by default
If you have passwords Xmarks isn't a webextension but you can use it on Firefox 56 and then upgrade.
https://forums.lastpass.com/viewtopic.php?f=45&t=266745&star...
This was a big relief for me, as it was the one extension I was really sweating on.
That said, I would love if there was a Chrome sync for Firefox, or Firefox sync for Chrome capability to sync bookmarks across them both.