In most browsers Resource Loader caches modules in LocalStorage in order to not rerequest the same modules on every page-load. However, this is not the case for Firefox, as Firefox used to apply the allowed storage quota per domain, not per origin. Thus, the use of LocalStorage was disabled for Firefox in T66721: mw.loader.store should not occupy all of localStorage.
However, with Firefox 69 this quota should now be per origin: https://bugzilla.mozilla.org/show_bug.cgi?id=1064466#c18. So it should be possible to reenable the use LocalStorage again for users with a current Firefox.
I'll provide a patch as basis for discussion.