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I have the same problem and updated a thread I started in 2016 where no one really provided an answer. I was using xmarks but the OSX client that was "supposed to" sync to Safari sucked and errored out all the time. Now Xmarks announced they are shutting down in May this year. I just want to use Firefox on the desktop and have my iPhone still have up to date bookmarks without having to use the Firefox browser on iOS (unless / until Apple ever lets you change the default browser which is unlikely). Why is this so hard?
 
Safari and Firefox can both export and import bookmarks as HTML.
You end up getting a page with a bunches of bookmarks on it, instead of a nice list of bookmarks in your bookmarks menu.
But you can bookmark that HTML page, and the bookmarks on it at least work in either browser.
This is yet another case where the developers are being intentionally stupid; likely under the direct supervision of management.
 
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