Using web-ext 3.1.0 w. node v10.16.0 (both freshly reinstalled, former globally).
$ web-ext --version
/usr/local/lib/node_modules/web-ext/node_modules/thenify/index.js:30
return eval(createWrapper($$__fn__$$.name, true))
^
SyntaxError: Unexpected identifier
at thenify.withCallback (/usr/local/lib/node_modules/web-ext/node_modules/thenify/index.js:30:15)
at /usr/local/lib/node_modules/web-ext/node_modules/thenify-all/index.js:55:65
at Array.forEach (<anonymous>)
at promisifyAll (/usr/local/lib/node_modules/web-ext/node_modules/thenify-all/index.js:53:11)
at Function.withCallback (/usr/local/lib/node_modules/web-ext/node_modules/thenify-all/index.js:33:10)
at Object.<anonymous> (/usr/local/lib/node_modules/web-ext/node_modules/mz/dns.js:2:24)
at Module._compile (internal/modules/cjs/loader.js:776:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:787:10)
at Module.load (internal/modules/cjs/loader.js:653:32)
at tryModuleLoad (internal/modules/cjs/loader.js:593:12)