Advice please Concrete5 v8.4.0 not fully working with Firefox 60.0.1

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Hi,

After fresh install of concrete5 8.4.0 RC4 with all requirements OK,
The dashboard file manager acts as if there is no javascript on the page if viewing with Firefox 60.0.1 64b
On the same laptop it works ok with chrome and ie11.
Tried with add-ons removed too. With antivirus off too.
On another laptop it works ok with the same FF.
We can't find no difference between the two configuations of FF.

Question : any tip to help investigate forward?

Seems there is no relation with
https://www.concrete5.org/community/forums/installation/files-manage...
or
https://www.concrete5.org/community/forums/installation/file_manager...