I started seeing this some time ago and finally decided to look it up.
I first went to https://developer.mozilla.org/en-US/docs/Tools/Network_Monitor/request_list but under Transferred it only states that:
Transferred: The number of bytes that were actually transferred to load the resource. This will be less than Size if the resource was compressed. From Firefox 47, if the resource was fetched from a service worker cache, then this cell displays "service worker".
Then I found https://bugzilla.mozilla.org/show_bug.cgi?id=1358038 via https://duckduckgo.com/?q=firefox+network+Transferred+"(raced)".
I read the entire thread but actually the first tree posts was enough:
The RCWN feature in Necko adds the ability to race the cache with the network when the cache is slow. So if reading from the disk is slow, we will send a network request, and return the channel from the network, even though we have the entry in the cache. This way we provide the content to consumers faster.
Our concern is that requests that load from the network instead of the cache will confuse users, and they will think there is a problem with their caching headers, or with Firefox's caching. We want to avoid that by showing them why it loaded from the network instead (because we raced).
This information should really be added to MDN and preferably not in a corner like https://developer.mozilla.org/en-US/docs/Tools/Network_Monitor/request_list, but at least it should be mentioned there. Actually, that would probably be the best place to explain the race concept, with a link from devtools to it. Alternatively it could be mentioned at https://developer.mozilla.org/en-US/docs/Tools/Network_Monitor/request_details?utm_source=mozilla&utm_medium=devtools-netmonitor&utm_campaign=default#Timings which there is already a link to in devtools (in the Timings pane after selecting a request in the network pane).