Hi,I will tell you the detail.
1. If you've recently deleted a large number of bookmarks, there is likely nothing to worry about. You can close this window and click "OK" to allow Xmarks to continue synchronizing.
2. If you haven't recently deleted a large number of bookmarks, something may have gone wrong with your bookmark set. Continue synchronizing. Before you continue synchronizing, we strongly recommend that you examine your bookmarks either by opening the bookmarks menu in this browser window, or by opening the Bookmark Manager by clicking "Organize Bookmarks" ("Manage Bookmarks" in Firefox 1.5) in the bookmarks menu. If everything looks okay, you can close this window and click OK.
WHAT TO DO IF YOU SEE A PROBLEM
If you do see a problem with your bookmarks, don't fear: your bookmarks are likely to be recoverable. Here are some things you can do:
Download your bookmarks from the Xmarks Server,and the alternative is to recover your bookmarks from Firefox's backup. Firefox automatically maintains a set of bookmark backups for you, typically 5 days' worth. Instructions for recovering from these backups are available on the Mozilla site.
After you recover your bookmarks, you should immediately perform an upload with the following steps:
1. Open the Xmarks Control Panel (Bookmarks -> Xmarks..., or Tools -> Xmarks... -> Settings under version 2.0.xx)
2. Click the "Other" or "Advanced" tab.
3. Click "Upload Now".
4. When a message appears asking you to confirm the overwrite of the server's bookmarks, click "OK".
If Firefox doesn't shut down properly, your bookmarks file on disk can become corrupted, for instance. There may be a problem with your profile, the set of files in which Firefox stores all of the information specific to your installation, including your bookmarks.