The localstore.rdf file used by Firefox browsers
aka...The Only Really Extremely Annoying Thing about Firefox Browser
In many versions of Firefox browser, one file can be quite easily corrupted so that any custom toolbar tools and/or bookmarks on your toolbar can suddenly disappear. Sometimes opening a new window is all it takes to cause the problem.
However, you might want to try an easier way.
A quick remedy is just keeping a backup copy of your "localstore.rdf" file, closing Firefox, and overwriting the localstore.rdf file in your personal Profile folder.
If you need help locating your Profile folder, here it is.
You can also create a "batch file" to automate the replacement process.
If your Firefox toolbars disappear frequently, you can (on Windows computers)
create a file which automatically replaces your corrupt localstore.rdf with your
back-up copy. Then just assign an icon on your desktop which, when clicked, overwrites
your corrupt localstore.rdf file with your back-up copy.
Click here for details on how to create the batch file.