looks like its just setting some events, these two lines should clear the anti-select and the anti-right click respectively if pasted into the debug console:
Firefox has an add-on called "Allow Right-Click" that lets you easily toggle blocking right-click scripts. Some sites offer a useful context menu, like Google Drive, so you don't necessarily want to always be blocking them. Hence the toggle.
you reminded me of a site that was "down for maintenance" (they were just spamming an alert) after using the block multiple alerts button in firefox, it works fine