On Linux, firefox uses xdg-open to determine how to open different mimetypes. You can check what is currently being used by running: $ xdg-mime query default application/pdf Then, assuming your app makes a standard .desktop entry in /usr/share/applications, you can set it by doing $ xdg-mime default <app>.desktop application/pdf If you need to manually set it, the file that sets these are in $HOME/.config/mimeapps.list

If you're referring to internally how firefox handles files, that can be changed in about:preferences .

$ xdg-mime query default application/pdf yields okularApplication_pdf.desktop, so that seems to be right. I've attached my settings in about:preferences as picture. So, what else could be the reason? (Actually, I would like okular to open pdf files but gimp is used instead)

The only suspicious entries in /usr/share/applications with gimp and pdf are (/usr/share/applications$ grep gimp * | grep pdf)

• gimp.desktop:MimeType=image/bmp;image/g3fax;image/gif;image/x-fits;image/x-pcx;image/x-portable-anymap;image/x-portable-bitmap;image/x-portable-graymap;image/x-portable-pixmap;image/x-psd;image/x-sgi;image/x-tga;image/x-xbitmap;image/x-xwindowdump;image/x-xcf;image/x-compressed-xcf;image/x-gimp-gbr;image/x-gimp-pat;image/x-gimp-gih;image/tiff;image/jpeg;image/x-psp;application/postscript;image/png;image/x-icon;image/x-xpixmap;image/x-exr;image/x-webp;image/heif;image/heic;image/svg+xml;application/pdf;image/x-wmf;image/jp2;image/x-xcursor;
• mimeinfo.cache:application/pdf=gimp.desktop;gv.desktop;libreoffice-draw.desktop;net.sourceforge.gscan2pdf.desktop;okularApplication_pdf.desktop;

Maybe it's the mimeinfo.cache entry. But why then the correct result of xdg-mime query?

I've changed the entry in mimeinfo.cache manually to

• application/pdf=okularApplication_pdf.desktop;gimp.desktop;gv.desktop;libreoffice-draw.desktop;net.sourceforge.gscan2pdf.desktop;

and that seems to solve the problem. But why the corret result of xdg_mime? And how would I have changed it the correct, i.e., tool way?

Please see the man page for xdg-mime; it describes what the query option actually does. I'm glad to see a manual edit fixed your issue.