My bookmarks have all suddenly become this: " live bookmark feed failed to load" all of them are the same, all my bookmarks are the same and the bookmarks I had before have all gone.

If I could trash firefox and start again but that is not possible, I have tried everything, reinstalling from an HTML file, etc. it will not even let me delete a single bookmark!

What to do, reinstall Ubuntu??

If you want to reinstall mozilla make sure to delete the .mozzilla hidden folder in user account or reinstall will get all the same set up and data back.  Something like this I would un install it completely go to user and delete the hidden file then re install it will make a new file on the first run.  This is why you sometimes get the same problems back after reinstall an app.  Not sure whats up with the book marks but would reinstall the browser before the OS.

Run firefox -profilemanager as root & and setup a new instance.

BR.JACK