It does nothing :(

[robozb]

[NathanJRollins]

Transferring a 5GB file across town last night, I noticed that Firefox appeared to do nothing, but after waiting a few minutes with progress displayed at 0%, the progress bar would jump ~25% at a time, with the sender always seeing the transfer one ~25% tick ahead of the receiver. The Edge browser, however, reliably updated progress constantly.

[kern]

Candidly, I've tested much more thoroughly with Chrome-based browsers like Edge than I have with Firefox. When I have a spare day or two to look into this I will!

[cablesky]

It's the same behavior in Chrome. Just tried it!