I too have been a long time user of Xmarks, but recently removed the firefox plugin from ff v56 (holding off on upgrading to v57). For me, with ff v56, it seemed to update the xmarks servers, but did not sync out to other Windows firefox machines. No need to endanger having something mess up my bookmarks, so I'll wait this out... perhaps forever.
My temporary (perhaps permanent) work around is to use firefox sync. I'm not much of a cross browser user, so staying with firefox and their bookmarks works for me.
I use the Xmarks ios app and this used to work fine but was in need of a major upgrade. That's how I'd open my ff bookmarks in Safari. But, recently I've found a work around. Now, I'm using ios Icab Mobile. It will sync with firefox bookmarks. This provides a more seem less access while browsing on my Iphone or Ipad.
Sure wish there was away to merge my Safari bookmarks with the firefox bookmarks. ICloud for Windows seems to support this... but I've yet to get this working. AnyTrans is another option that I not tried.
If anyone out there finds a way to easily merge Safari bookmarks and firefox... please announce it here.