Progress bar doesn't update on Firefox if game.data set to preload

[FormularSumo]

Both Chrome and Firefox call onprogress repeatedly throughout loading, however for Chrome if (event.loaded) is returning true while in Firefox it's not.

If I force the code to run regardless of whether it's true, Firefox gets the correct total figure but shows loaded as 0 for the duration of the loading screen..

Outputing the onprogress event shows these differences

Or in more detail

[FormularSumo]

I have reported this issue on the Firefox bug tracker - https://bugzilla.mozilla.org/show_bug.cgi?id=1918728 - as it seems to be caused by a bug in the browser.

Other things I could potential try to work around it include using Fetch instead of XMLHttpRequest, or try to get the progress of the preloaded request directly instead of XMLHttpRequesting it again.

[FormularSumo]

Have tried looking into getting the progress of the element but it seems like the only way to do that is through requesting the object - which is what I'm already doing through XMLHttpRequest