Firefox 51 (actually, any version), English, Linux, OpenSuSE Tumbleweed (and 42.2, at least).
DTA shows locations of the local files as ///path/to/file/or/directory, so when it is then tries to open it in a file manager, it fails. It fails, for example, in Dolphin and SMPlayer.
To my opinion, correct schema should be either file:///path/to/file/or/directory or /path/to/file/or/directory.
This issue first appeared about 4 months ago.
File or directory opens.
File manager complains about the missing file
Save a file, try to open it in Dolphin.
Please enable Diagnostic Logging in the Privacy tab of the DownThemAll! preferences and either copy/paste essential part of the log or attach the entire log. Please note that the log may include sentitive information which you have to remove prior to posting it!