No they do not need to be online at the same time. However it would be much better to manually transfer bookmarks. Sync is not always reliable especially with large differences. (See lower down. - .json ) Even if you use Sync backup the bookmarks manually on both computers first, or don't complain if it messes up. #If the New Windows 7 machine has only a few new bookmarks export them as HTML. <br /> See [[Export Firefox bookmarks to an HTML file to back up or transfer bookmarks]] #* If it has a lot of bookmarks. Stop & think. This list not applicable: Use Sync; after making backups; and keep fingers crossed !! # Make sure syncing of bookmarks is off. <br /> [[How do I choose what types of information to sync on Firefox?]] #Then backup the XP bookmarks as .json backups # Restore the XP old bookmarks to the New Windows 7 machine. Now both machines have the same bookmarks. # Note HTML files are pretty easy to edit if that simplifies things, by removing duplicates. ## Import the HTML listing back into the New W7 machine, that now has a full set of bookmarks. Although the imported ones will now be at the bottom of the listing. ##Import the same HTML listing into the XP also. <br /> [[Import Bookmarks from an HTML file]] #* You should now have Matching full sets of bookmarks. # Re enable Sync of Bookmarks. Future changes will be sorted out by Sync. ------- I would '''strongly recommend making manual backups before first using Sync to sync bookmarks.''' I would also recommend making regular manual backups whilst Sync is in use for bookmarks. * [[Restore bookmarks from backup or move them to another computer#w_manual-backup]]'''#w_manual-backup''' {File type '''.json ''') At first thought it seems totally unnecessary to use sync and resort to backup bookmarks manually, after all the syncing effectively backs up bookmarks to multiple devices. The reasons for this counter intuitive suggestion are that Sync * Now has only one option, merge. It will not do uni-directional syncs. It will attempt to make all bookmarks libraries identical. * Sync is not totally reliable. Problems especially occur if there are slow internet connections and particularly if there many changes to bookmarks. * Failure results in duplication &/or other corruption of the bookmarks libraries of the devices being synced. The task of sorting out the bookmarks after corruption is probably not feasible. All you can do is resort to whatever backups you have.