This functionality does not exclusively depend on firefox but also on the server to support something like the Range Request https://devdoc.net/web/developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Ranges.html If the server does not provide the functionality, there is not much firefox can do except re-download/request the entire file, when your retry.
But it might be nice if firefox could check before downloading if a server supports something like the Range header and better indicate what a pause/interrrupt/retry means.
A brief warning that auto hides itself after a few seconds when a server does not support resume, might be nice so people can immediately cancel if they dont want to risk having to re-download or interrrupting because they are currently on wifi or something.