"Increases the time items will be stored in the browser cache. This increasing the chance that network resources won't need to be fetched. As NVMe/SSD storage is cheap and lightning fast compared to network requests, this has the potential to improve load times significantly".

This extension is very effective to reduce bandwith usage (useful when using tethering, for example).

Advice to make it more effective:
- Make sure you have disk cache enabled.
- You can also increase disk cache size to benefit more (1)
- You can check the +6 month expiration date in about:cache?storage=disk

(1):
If you want to force disk cache to 4 Go, in about:config :
browser.cache.disk.smart_size.enabled = false
browser.cache.disk.capacity = 4000000 (cache up to 4 Gbytes on disk)
browser.cache.disk.max_entry_size = 1000000 (cache objects up to 100 Mbytes)

It's working.