Several weeks after writing my comment from January 3rd I discovered a bug report related to this problem:Pepitaux wrote: Here is the problem: I display any jpeg (extension .jpeg or .jpg) image found on Google, I drag and drop it on my desktop, and 99% of the time the default app to open the image has changed (in my case, from "Preview'" to "Neat Image").
Bug 1389836 "Saving images via drag-and-drop loses filename extension".
Then, about two weeks ago I noticed that the file/application association was broken for all my text files. TextEdit.app had been for years the default app for my .txt, .rtf, .ini, .mab, .css or .db files but suddenly for an unknown reason this association was broken and TextEdit was replaced with Pages or Numbers as the default app.
Restarting my Mac or using the "maintenance" feature of Onyx or a special command line in Terminal) would temporarily restore the text file/TextEdit association but half an hour or several hours later the association was broken again.
It took me several days of searching and a ten days discussion in a French macOS newsgroup until I found the following Apple discussion:
"File Associations Keep Changing" https://discussions.apple.com/message/33052869
Finally a more thorough search made me discover the more recent Bug 1437281 "OSX dragging image to desktop changes OSX File associations"
Comment #22 describes a workaround for this issue until the bug gets fixed:
• type about:config in the URL bar and hit Enter,
• search for the option "security.sandbox.content.level" and toggle its value from "3" to "2".
• Restart Firefox.