Hmm, that’s puzzling because there’s a help article about it:
Try this to re-enable the missing menu item:
(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.
(2) In the search box in the page, type or paste import and pause while the list is filtered
(3) Double-click the signon.management.page.fileImport.enabled preference to switch the value from false to true
(4) Open the Logins and Passwords page or, if it’s already open, Reload it
This was a backup you made yourself from your old Firefox? Did you use the “Restore” function? That is the only one that can read JSON; the Import function only reads HTML.
You’ll need to use the “Choose File…” option and point Firefox to your backup file (any backup file already listed on the Restore menu is from the new installation).