You can use sync to share selected data between different profiles/browser versions/and systems. You can use sync to share passwords, bookmarks, and history. Anything else may not be compatible between different browser versions or computer systems. https://support.mozilla.org/en-US/kb/how-do-i-set-sync-my-computer Sync your bookmarks, history, passwords, add-ons and open tabs with another copy of Firefox. Learn how to set up Sync. https://support.mozilla.org/en-US/kb/how-do-i-choose-what-types-information-sync-firefox We'll show you how to choose which types of information (bookmarks, history, add-ons, and passwords) to share across all your devices through Firefox Sync. https://support.mozilla.org/en-US/kb/firefox-sync-troubleshooting-and-tips https://support.mozilla.org/en-US/kb/sync-firefox-bookmarks-and-browsing-history-iOS Sync your passwords, history, tabs and other browsing information across your iOS, Android and desktop computers with Firefox Accounts.

When you import bookmarks into Firefox using an HTML file (sometimes called bookmarks.html), they are added but Firefox doesn't remove duplicates. Hopefully Sync with do a better job, but if it also causes duplication, you might need an add-on to help clean up.