I finally found a solution to this issue here. It was option 2 in Ronald P's answer.
Basically, somehow my
~/.local/share/applications/opera.desktop file got messed up:
[Desktop Entry] Encoding=UTF-8 Version=1.0 Type=Application Name=(72) Facebook - Opera Icon=opera Exec=/usr/lib/x86_64-linux-gnu/opera/opera StartupNotify=false StartupWMClass=Opera OnlyShowIn=Unity; X-UnityGenerated=true MimeType=x-scheme-handler/unknown;x-scheme-handler/about;x-scheme-handler/https;x-scheme-handler/http;text/html;
%U to the end of the
Exec= line fixed it. (And, of course, I got rid of "(72) Facebook - " in the
Name= line. How the heck did this happen??? Dunno, but this is how to fix it in case anyone else has this problem. Man, I tolerated this problem for MONTHS! Glad it's finally fixed.