You can use the search bar in the library (Bookmarks > Show All Bookmarks) and enter a common symbol like a ":" in the search bar. You can either search the history or the bookmarks.

You can also export the history with this extension:

• History Export: https://addons.mozilla.org/firefox/addon/history-export/

Then you can import them in another program and sort them there.

You can also export the bookmarks to an HTML file in the library.

This may sound dumb, but so what — I did try & look a bit before posting this — What is a good program to import the History Export file into, in order to be able to sort/alphabetize/find duplicates?

You should be able to import the text plain version of the exported bookmarks in a spreadsheet program and do the sorting there.