Firefox 55 has introduced a regression where uploading a file from web forms doesn’t work in certain circumstances due to a change in how upload stream is handled, likely leading to a 400 Bad Request error response from the server. Users are reporting that they cannot upload video thumbnails on YouTube which is offering an Ajaxy form, but static forms may also be affected. Mozilla developers are working on the solution.
Update: This regression, turned out to be a bug involving service workers, has been fixed with Firefox 55.0.3. However, there’s another upload regression in Firefox 55 where async
fetch using a
FormData with certain
Blobs will fail even without service workers. Mozilla developers are working on this bug as well.
Update 2: The
FormData regression has been fixed with Firefox 56.