On a ubuntu 18.04 machine, the asking dialog for save file (The dialog to choose what to do for downloading files, open or save) hangs for a while. can not choose between options and the OK button is freezed, until at least 30 seconds. I have this problem since I witched my system's hard to an SSD and re-installed ubuntu. safe-mode does not show any error or problem while I have this problem.
For me, this ended up being an issue with Firefox 65. It was tracked in Bug 1517101 and fixed in Firefox 66 (released very recently). I would double check that you're running on Firefox 66 and retest. Hopefully you're experiencing the same problem in that bug.