So, somehow my default web browser got changed from SeaMonkey back to Firefox. (Bleh!) I ended up cleaning up after xdg-open in the process of correcting my default web browser. In ~/.local/share/applications/ (where user-customized .desktop files go), there were several files in the format userapp-<appname>-<short_hash>.desktop. userapp-Firefox-FIK3WX.desktop userapp-Pale Moon-1HNY8X.desktop userapp-SeaMonkey-YV09AY.desktop userapp-Thunderbird-PIZCZW.desktop (Wow! I have gone through a lot of web browsers lately.) I remember creating a few .desktop files in this directory, but I don't remember creating any userapp-* files. The contents of userapp-Firefox-FIK3WX.desktop were: [Desktop Entry] Encoding=UTF-8 Version=1.0 Type=Application NoDisplay=true Exec=/usr/lib/firefox/firefox %u Name=Firefox Comment=Custom definition for Firefox This is weird. There is only an English comment (the default has many languages), and this one has set NoDisplay. I deleted all my custom .desktop files. I have no use for them anymore. And back to the problem of default web browsers. These userapp .desktop files got into mimeapps.list (now located in ~/.config/). I suppose the short hash for SeaMonkey changed, causing mimeapps.list to fallback to the next web browser listed in mimeapps.list. I had to weed all the userapp garbage out and replace them with the system-wide .desktop file names. What daemon creates these mysterious userapp .desktop files? I don't want this daemon creating this junk on my system. --Kyle