Why can't Confluence import files using recent Firefox updates?

[Tina Hutchinson]

Why are recent Firefox releases (55.x) not working in Confluence when trying to import a file? After browsing to the file and clicking "Insert" the progress bar just hangs, instead of inserting the file to the page.  Same thing if you try to drag & drop, or upload using a File List.  The files won't import. This happens with any type of file- Word, PDF, mp4, etc.  I had to reinstall an earlier version of Firefox (52.3), so the file import process works again. We use Confluence Cloud.

[Gonchik Tsymzhitov]

I think it's too obsolete