After upgrading both FF to 56.0.2 and Xmarks to 22.214.171.124, and both on Win7 and Mac OS X, I found both FFs stopped to work anymore.
1. The first run of my FF on Win7 asked to "merge" bookmarks , and created lots of redundant empty folders.
2. I removed them and re-sync them again in Chromes on both Win7 and Mac OS X.
3. Then my FF on Win7 complained to have sync problems.
4. I tried to download bookmarks directly from the server, the usual way to fix issues.
5. Now the full download simply failed. All bookmarks on FF were cleaned up but I could not get any booksmarks ever since.
I have tried every suggestion but there is no way to fix this new version.