Translated by DeepL. Sorry about the possible nonsense:

It works fine, but why can't it be used with Firefox Sync active? FF Sync is essential for many people who use tab synchronization, password synchronization, history, sessions, plug-ins... I don't want to give up all that just to be able to synchronize my bookmarks, especially when I can also synchronize them with FF Sync, I just prefer to have them in my private cloud.
Please correct that important flaw, because it's a shame we can't have the best of both worlds.

Greetings

"Do one thing and do it well"
This add-on does what it's advertised to do, very easy to setup and uncomplicated.

Habe leider ähnliche Probleme wie andere Nutzer hier, versuche die Lesezeichen mit meiner NextCloud zu synchronisieren und es kommt der Fehler "Invalid bookmark: {"guid":"unfiled______}"

Gibt es hier keinen Workaround oder liegt die Ursache wirklich beim Autor der bookmarks App für Nextcloud?

EDIT:
Danke ich habe den Tipp mal versucht und versucht nur "Lesezeichen-Menü" zu synchronisieren, dann kommt aber nach einer Weile der Fehler "Trying to create a URL that is already bookmarked. This shouldn't happen! Please tell the developer about this!"
Offenbar wurde hier ein Teil doch schon synchronisiert, finde in NextCloud aber nicht heraus, wo ich diesen löschen kann...

Installed Floccus and registered an account in OwnDrive to backup bookmarks. The addon never synced.

At the left of the "Options" and "Sync Now" buttons it says "X Error!" and immediately below the buttons an error description says "JSON.parse: unexpected character at line 1 column 1 of the JSON data".
No idea what to do, error is not self-explanatory and does not give any hints regarding which JSON is crippled, nor a dump of such file; checked the GitHub site but could not make heads nor tails of the code (not a JS dev, more of a C# and VB.Net one I am).

Checked account "https://my.owndrive.com/" with my username and password and it browses OK. Would like the author to try an OwnDrive account, as they say it supports NextCloud, and post his findings, or at least a basic tutorial on how to set up Floccus with a third-party server, not just with localhost. I am not really sure of the URL to use; tried several (taken from the site being browsed in Firefox) to no avail. I'm sure I'm the one doing something wrong, but there are no good working examples of Floccus setups in the Mozilla not Addon Home pages.

Using FF 59.0.2 (64-bit) on Ubuntu 16.04.

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EDIT 1: Cannot reply to your reply, so using this edit.
Thanks a lot for the response.
No, I did not enable or install a Bookmarks app, and my guess is that I would have to do that on OwnDrive.
OwnDrive has its own applet that one would drag to FF Bookmarks to upload, but I guess that is not the one you refer to. Also, no versions given.
Can you help me providing an adequate Bookmarks App link? If it has install instructions for third-party servers like OwnDrive, much better.

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EDIT 2: Cannot reply to your reply, so using this edit.
Also tried Wolkli, does not even have a Bookmarks secction, looks like they removed it.
None of OwnDrive or Wolkli allow adding apps to their free accounts. So the addon isn't any good to me. It could work for people that have their own NextCloud server, but not my case. Too bad.
In view of this, I'll give 4 stars to the addon.

Thanks for your great work so far.
Gave you some little support on liberapay - and this is what other people explaining here should do as well - this an open source application developed by one guy in his freetime. I value that.

Firefox Android Support
The main feature currently missing for me is the firefox android support. This hasn't been done yet because of missing support of the API in firefox android. If anyone like me wants to speed up this feature - he can donate here:
https://www.bountysource.com/issues/56793689-implement-bookmarks-on-android

Link to bug in Floccus on github:
https://github.com/marcelklehr/floccus/issues/3

Link to bug for firefox android (Bugzilla):
https://bugzilla.mozilla.org/show_bug.cgi?id=1330160

Nach anfänglichen Schwierigkeiten läuft die Erweiterung inzwischen Perfekt und ist unverzichbar für mich! Herzlichen Dank nochmal für den tollen Support und die unglaublich schnellen Versionsupdates :-)

Thanks for adding folder/tag support! Firefox Bookmark sync has been iffy and XMarks really spoiled me until it went down the tubes.

Beim Versuch meine Bookmarksammlung mit Floccus und Nextcloud zu synchronisieren, bekomme ich eine völlig unverständliche Fehlermeldung:
Error! invalid Bookmark: {"guid":"mobile______"}
Diese Art von geheimnisvolle Fehlermeldung, die nicht zurückzuführen ist auf ein konkretes Lesezeichen, ist ein absolutes NoGo. Eine Fehlermeldung, die nur für den Programierer zu deuten ist, hat in ein Userprogramm nichts zu suchen. (Auch ein "invalides" Lesezeichen liegt in klartext vor, und kann als solches gezeigt werden!

Great app, Would love to see some Tag to Folder translation That way it becomes a whole lot more manageable with 500+ bookmarks. Multiple tags is the same bookmark in multiple folders. Works perfect in combination with Passman :)

Thanks to you, this add on is really convenient

Works great. really missing tag sync, but we'll need to wait for that.

If you have access to a Nextcloud server with the Bookmarks app installed, and provide Floccus with your credentials it syncs, nice! I appreciated the robustness of being able to move my Nextcloud bookmark sync folder from Other Bookmarks to the Bookmarks Toolbar and still having it sync.

Would love to see two features: propagate tags and descriptions both ways, as well as a push sync to this client when you save or edit a bookmark in Nextcloud.

Funktioniert ganz gut . Wünschenswert wäre noch die Synchronisation der Tag´s und auch eine Version die mit Firefox für Android funktioniert. Aber mir ist klar das das eher ein Problem vom FF ist . Da diese Funktionalität in aktuellen Versionen nicht vorgesehen ist.
Darum auch 5 Sterne .

Very nice plugin ! and the only one that can sync bookmark between Firefox and Netxcloud (last version 12 at home !)

This is the first version that work (almost) fine on my computer ! I have again some errors in sync but it's a good start...
Domage that i can't sync bookmark on my personnal bar - it's the place where i have all my bookmark :-(

May be in a next version ?

I'll gave you a bear for this job ;-)

Fill in credentials, hit the 'Save' button, nothing happens at all. Pretty useless.

Does not work for me. I fill in the settings and hit save. Nothing happens.

Running Firefox 56.0 on Linux Mint 17.1

this one works very well

works very well. Just does what it's meant for! Good work.

La synchronisation n'a l'air de marcher que dans le sens owncloud->firefox... Si cela est, il serait bon de le préciser. À utiliser donc avec l'applisignet... Envisagé comme ça, cet addon peut donc être utile ! Je remonte ma note en conséquence....