I bookmark a page. It stays while I'm still working on the computer, but after I,m done and computer goes into sleep mode and then I return it doesn't show up where I saved it. and it doesn't show up in the recently bookmark menu.
this can be a problem with the file places.sqlite that stores the bookmarks and the history.
Check this: Bookmarks not saved
But also this might be of some help: Can't add, change or save bookmarks - How to fix
Additionally, check this article: Preferences not saved
Hope that helped you! Please report back!
You can check for problems with the places.sqlite database file in the Firefox Profile Folder.
Does the new bookmark stay if you do a backup of the bookmarks? I found it a good idea to save a backup copy of the bookmarks outside of the profile just in case something happens to the profile.