Webgl builds stopped working - windows, ffox

Target Platforms Web
1

using 5.5.1 and firefox 53.0 (windows10) and my latest webgl builds will not get past loading screen - if I do a development version this is in the console log:

Successfully compiled asm.js code (loaded from cache in 983ms) 204c240b-e323-4a5b-a8b6-c893870dec82
run() called, but dependencies remain, so not running UnityLoader.js:3545:2
Module.printErr http://localhost:50455/Development/UnityLoader.js:3545:2
run blob:http://localhost:50455/1e7a57b1-b5a2-4f1d-bb8b-bcc04a145395:406208:3
blob:http://localhost:50455/1e7a57b1-b5a2-4f1d-bb8b-bcc04a145395:406296:1
unreachable code after return statement[Learn More] 1e7a57b1-b5a2-4f1d-bb8b-bcc04a145395:19139:1
Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user’s experience. For more help http://xhr.spec.whatwg.org/ 1e7a57b1-b5a2-4f1d-bb8b-bcc04a145395:154:2
XML Parsing Error: syntax error
Location: http://localhost:50455/Development/webgl.js
Line Number 1, Column 1:

2

Is this an empty project?

3

Nope, BUT in fact I tried an empty project (a cube only) in 5.6 and much the same thing happened with the web console open with a dev build.

It is ok without dev build ticked so I guess the compiler just complains a bit about that version for some reason!