Thanks for the feedback! An option to opt-in for syncing is a feature that is definitely on my radar. Unfortunately, the tools Firefox makes available to extension developers impose a size limit on the amount of data that can be synced, and also do not make any promises regarding sync speed and frequency. At this moment these drawbacks prevent me from implementing this feature.

P.S. Please do not rate down an add-on based on things it doesn't claim to do. Add-ons should be judged based on what they do claim to do and how well they do it. If you find a bug or wish to voice a suggestion please file an issue here: https://github.com/rharel/webext-private-bookmarks/issues I review suggestions there regularly. Thanks again for your feedback, it is appreciated!