Hello,

preferences affects download dialog boxes. you should check the preference in about:config and ensure the option which is shown in attached picture whose value is 1000 (or 0 for no delay) and not something much higher. if this setting is normal, the problem is something else, such as the communication setting up the download is taking long.

Try clicking on the Ok button anyway. Next, change the action selection. Then change it back.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?