The problem was finally related to Firefox Quantum:

« The new Firefox 57.0 Quantum with optimized performance setting quickly eats License Units

latest Firefox 57.0 Quantum, released 11/14 when installed on the box with Ensemble/HealthShare is eating up all available license units, eventually resulting in "license limit exceeded". 

That seems to be caused by the Firefox browser engine which is using parallelism optimizing the multi-core CPUs.

Here are the relevant links: 


There is one setting Performance in the latest Firefox about:preferences where the hardware acceleration + content process limit could be set. »