and in which unit is the default cache capacity size? mb or kb?

See:

• https://searchfox.org/mozilla-release/source/modules/libpref/init/StaticPrefList.yaml#999

# Disk cache capacity in kilobytes. It's used only when
# browser.cache.disk.smart_size.enabled == false
- name: browser.cache.disk.capacity
  type: RelaxedAtomicUint32
  value: 256000
  mirror: always

See:

• https://searchfox.org/mozilla-release/source/modules/libpref/init/StaticPrefList.yaml#999

# Disk cache capacity in kilobytes. It's used only when # browser.cache.disk.smart_size.enabled == false - name: browser.cache.disk.capacity type: RelaxedAtomicUint32 value: 256000 mirror: always

1. from when is this? when was it last updated? 2. you didn't answer my question properly with this copy pasted info.

You may deduct from the comment in the source code that setting browser.cache.disk.smart_size.enabled = true enables Firefox to automatically determine the size of the cache. There is no need to modify browser.cache.disk.capacity, merely flipping browser.cache.disk.smart_size.enabled is sufficient to enable/disable this feature.

You may deduct from the comment in the source code that setting browser.cache.disk.smart_size.enabled = true enables Firefox to automatically determine the size of the cache. There is no need to modify browser.cache.disk.capacity, merely flipping browser.cache.disk.smart_size.enabled is sufficient to enable/disable this feature.

and what about the ram? isn't there a smart size preference for the ram?

For the memory cache you need to modify the pref directly as you can see if you read the lines about browser.cache.memory.capacity right below the disk cache pref.

# -1 = determine dynamically, 0 = none, n = memory capacity in kilobytes.
- name: browser.cache.memory.capacity
  type: RelaxedAtomicInt32
  value: -1
  mirror: always

so you're telling me that for the disk cache there are 2 preferences, a preference for the normal disk cache capacity(browser.cache.disk.capacity) and a preference for smart disk cache(browser.cache.disk.smart_size.enabled) to let firefox dynamically determine the disk cache size while the memory cache has both of them in one preferences like the line you provided does say?

You can see the effect of the settings by visiting about:cache in the address bar.

See also bug 1027944.

• 1027944 - When disk cache is disabled, hide "disk" cache section in about:cache (otherwise it repeats "memory" entries for "disk" category)