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.

Simple, yet effective.
Sometimes I need files to be cached because I have slow internet, and this has saved me a few times already.