yes, i too am having exactly the same problem with firefox quantum (v57+)
before quantum, extensions were working fine, and continued working. but i deleted that firefox profile, and now i cannot install/add any extensions, to a new profile, using either firefox-esr or firefox.
i've been able to install *one* extension to waterfox, but cannot install any other extensions... ?
note that the only way i have found that does work, is downloading the .xpi from addons.mozilla.org, then adding it from about:debugging - however this is not safe because:
"The new temporary add-on feature bypasses the add-on signing restrictions that are in place in Firefox allowing you to load unsigned extensions in the browser."