The Eversync sync system does not work reliably with large sets of Firefox bookmarks. It tends to auto-duplicate entire folders, and even if you add extra space, this duplication pattern is repeated on each additional sync. The limit for successful sync is also much lower than the supposed 1500 folder limit. The system is also very resource-consuming, perhaps through poor design. So, practically speaking, if you have lot of bookmarks, at present this add-on is useless.

I will return to Eversync when I see 5-star ratings being given consistently. Then I will be confident that they have fixed the various problems. However I am skeptical because my impression is that they may be under-resourced in terms of programmers, and there may also be an issue of programmer quality. Possibly they are reckoning that enough people will pay and so give them a window in which they can finance extra programmer resources. Is that a viable strategy? I don't know. Meanwhile they are polite and quite nice to deal with, so they get 2 stars from me rather than 1.