Can only delete one URL at a time from History, why not more?

How are you trying to select multiple history entries? In the Library, you should be able to

• Right-click an entry and choose Forget About This Site to delete all history entries from that domain.
• Hold down the Ctrl key and left-click additional items to select multiple entries.
• Hold down the Shift key and click either higher or lower in the list to select multiple entries.
• Hold down the Shift key and press End to select all entries below, or hold down the Shift key and press Home to select all entries above.

If the above don't work for you, check if you have the same problem in Safe Mode.

• Use Troubleshoot Mode in Firefox

In the sidebar you can left-click at the left of the icon (favicon) to prevent opening this item in the current browser tab.