You can no longer access or search for keywords in the Bookmarks Manager (Library) as that column has been removed.
You will have to search for the title or URL to locate this bookmark to inspect and modify the keyword.

Note that if you start the keyword with a '@' then it will show up in the list of search engines in the location bar along with the builtin default search engines if you initiate a search via the '@' operator.

• https://support.mozilla.org/en-US/kb/address-bar-autocomplete-firefox

Hi goprivacy+mozilla, as a general rule, the URL for a "smart keyword" bookmark contains %s so you could try this:

• call up the Library window and focus it on Bookmarks ("Show All Bookmarks" or Ctrl+Shift+b)
• in the tiny search box at the upper right of the Library window, type %s

Firefox should show only those bookmarks in the right pane. When you click one, you'll see a form to edit its details at the bottom of that right pane.

I realize the results still lack some context, such as the parent folder, but... one problem at a time. ;-)

I just exported them from another computer where the Firefox profile did still show the keyword search bookmarks, and imported them into the ailing profile. This let me modify the affected bookmark effectively.

I'm just guessing here, but some sync action might have purged the bookmarks while still leaving the searches as state in a SQLite database in the profile, allowing me to continue performing keyword searches, but denying me the ability to actually manage them.