Normally, Firefox will capture the icon when you visit the site. Does that work if you use the bookmark? (Unfortunately, the icons themselves do not Sync, they have to be captured on each Firefox installations individually.)

If you use Sync then best is to disconnect Sync temporarily during troubleshooting. this could be a problem with the <b>places.sqlite</b> and <b>favicons.sqlite</b> files in the Firefox profile folder. *use the "Places Database" -> "Verify Integrity" button on the "Help -> More Troubleshooting Information" (about:support) page If errors are reported with "Verify Integrity", close and restart Firefox or reboot and retry. If "Verify Integrity" cannot repair places.sqlite, rename/remove all <b>places.sqlite</b> and <b>favicons.sqlite</b> files in the Firefox profile folder with Firefox closed. Firefox will rebuild places.sqlite and restore the bookmarks from the most recent JSON backup in the bookmarkbackups folder. *keep a backup copy of places.sqlite in case a new places.sqlite database has to be created *you lose the history when a new places.sqlite is created You may have to restore an older bookmarks backup in the bookmarkbackups folder via the Bookmarks Manager (Library). *Bookmarks -> Manage Bookmarks -> Import & Backup -> Restore *https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them See also: *https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional *https://support.mozilla.org/en-US/kb/cant-add-change-or-save-bookmarks You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the <b>about:profiles</b> page (Root directory). *Help -> More Troubleshooting Information -> Profile Folder/Directory:<br>Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder *https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data