Hi!

Nothing should happen. Your profile will be safely migrated. In any case, if you want to be on the safe side, you can always create a back up of your profile by following the simple instructions explained in this article: Back up and restore information in Firefox profiles

Thank you for a quick and informative response.

That definitely should take care of bookmarks: I actually see a sub-folder marked bookmarkbackups. However, I am still concerned about my "Library"/History of 3months. Do you know the name of the subfolder or file for my URL history? I would rest easier if I could actually find it?

Only program files are updated as a rule, the profile and its data are left intact. When you migrate to new versions, the way history, bookmarks and other data are handled and saved is the only time that some files in the profile are also updated. But the data itself is left as-is.

You can make a backup copy of data that you value.

• Bookmarks and History: places.sqlite
• currently open tabs: sessionstore.js

You can export (backup) bookmarks, but Firefox doesn't have a way to backup the History.

• http://kb.mozillazine.org/Backing_up_and_restoring_bookmarks_-_Firefox

• https://support.mozilla.org/kb/Recovering+important+data+from+an+old+profile

MozBackup will backup your bookmarks, history and most all other data, etc.