Recently, I have found that “Okular” and “Mount/Unmount Image” are the first choices in File Associations even for unrelated file extensions. I suspect that some Preferences-> Applications setting in Firefox (which is rapidly becoming unusable for many reasons) must have overwritten the defaults in KDE System Settings.
How might I restore the default file associations in KDE System Settings?
Try deleting ~/.local/share/applications/mimeapps.list and ~/.config/mimeapps.list.
If this doesn’t fix the issue then it’s a global issue caused by one of the packages you have installed recently.
The global mimetypes file is under /usr/share/applications/mimeapps.list,but I’d suggest figuring out which application changed the mimetypes and either remove that application or applications, or edit thier .desktop files otherwise this will happen again when the the mimetypes are updated again.