Comment 8 for bug 1951210

This issue still exists. The cause of the problem is that libreoffice generates a temporary HTML page somewhere under the system-wide /tmp, which contains a bit of javascript that redirects to file:///usr/share/libreoffice/help/index.html. But the firefox snap, being strictly confined, cannot see files under /tmp.

And apparently the reason for this convoluted solution is that xdg-open doesn't support URL parameters with file:// URLs (https://github.com/LibreOffice/core/blob/8cdfd9edce38447a3ab7660a0d534982bd9e69b1/sfx2/source/appl/sfxhelp.cxx#L942).