Problem was initially found with Teamspeak 3 links that wouldn't open in Firefox but in whatever xdg-open thought was the proper program (web links opening in Gwenview, or Kate, instead of Firefox). I thought it was the TS3 wrapper used to fix a problem on KDE so I modified it to use directly Firefox instead of xdg-open. It fixed TS3 links problem, but then the issue continued from within other programs. The real fix was to set the default web browser from terminal with the following command (the second command wasn't necessary, but it doesn't hurt to add the mime type). I think a KDE update messed that setting, as it was working properly before.
xdg-settings set default-web-browser firefox.desktop
xdg-mime default firefox.desktop x-scheme-handler/https x-scheme-handler/http