I am using floccus to syncronize my bm on both vivaldi and firefox, it took me a bit to figure out, but works fine.
There is another extension that does something similar
this one does not syncronize bookmarks, instead it is a client, you save and search bookmarks straight to/from the cloud.
In theory it works, but it is not mantained, so, does anyone know about some other extension that does that? a client rather than a syncronizer.