Saving from Firefox - problems with image, PDF, and video files

[chatan]

I can't be precise, but this has been going on since sometime last week, I think.

I've been having similar frustrations.    Hit and miss; inconsistent. 
Firefox will ask me where to download (per my preference setting); I select the folder (I've tried many): hit the Save button (it highlights) the "Save as.." screen disappears, but no download starts.

I first thought my old mouse was having a problem, so I bought a new mouse, with no change.

I tried the new profile, but it acts the same.

Very weird.