How do I I limit Firefox cache size using group policy ?

This may not be a parameter that is tunable through group policy.

The name of the Firefox about:config variable is:

browser.cache.disk.capacity

The value is dimensioned in KB.  The maximum value is 1048576 = 1024 MB.

You could try adding a new key and value in Group Policy and see if Firefox respects it.

See also:

https://support.mozilla.org/en-US/questions/1270059

Well, I find it quite remarkable that this value is not tunable via the GPO being provided by Mozilla.

You could try adding a new key and value in Group Policy and see if Firefox respects it.

How would I go about it (I understand I have to turn "off" smart management (which with my users allow the cache to grow completely out of reason, up to filling up the entire C drive...) and then set a specific value  (ie a "two-prong" action).

the browser cache will not fill up the entire C drive it is limited to 1024MB

the browser cache will not fill up the entire C drive it is limited to 1024MB

Unless you are running a RDS host in which case each user will have 1Gb of cache - not to mention that I think this does not include HTML5 "local storage". Yes we are redirecting the AppData so it is not really filling up our whole C drive, but still a very significant chunk of data that we would like to be on charge of...

You must be careful attempting to manage HTML5 local storage.

If you set the value lower than what's required, this can cause sites using local storage to fail in very subtle ways, as data becomes corrupt.

HTML5 Storage != Browser Cache

These are 2x completely different storage mechanisms.

agreed and fully understood

still want to keep things under control - I know web browsers are tuning into full-fledged OSes but we are still trying to keep them under control, despite the tide.