Whenever I open a dialog in Firefox the dialog in question is closed all by itself. I.e Firefox closes this dialog within less than a second or sometimes after a little longer (a few seconds up to 10 seconds). So, sometimes I manage to upload a file, if it located on the desktop. There is, however, no point browsing for a file from within the dialog as Firefox will close before I can find the file.

Go to a site with some contents to e.g. be printed. Open the print dialog, and just wait for a short while, often a second (sometimes less) or so, and the dialog disappears all by itself.