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.
Too many websites including ones charging high fees have unstable downloads that can be resumed, but only if you restart the download near instantly... Where one of these silly restarts occurs and a large file is nearly I would like Firefox to prompt whether I wish to restart, or resume.