Are the files HTML documents? If so, what setting do you have for HTML document in Settings → Files and Applications → Applications. There are also applications File and plain text document.

Right, what are you trying to open? see screenshots

Operating System: openSUSE Tumbleweed 20240208 KDE Plasma Version: 5.27.10 KDE Frameworks Version: 5.114.0 Qt Version: 5.15.12 Kernel Version: 6.7.4-1-default (64-bit) Graphics Platform: X11 Processors: 12 × Intel® Core™ i7-9850H CPU @ 2.60GHz Memory: 125.1 GiB of RAM Graphics Processor: Mesa Intel® UHD Graphics 630 Manufacturer: HP Product Name: HP ZBook 17 G6

Thanks for the reply!

Yes, the files are HTML documents. It seems to come down to the protocol.

     gio open http://www.freedesktop.org/

opens in Firefox, but

   gio open file:///home/tim/perl/foo.html

opens in Chromium.

Firefox is happy with the HTML files. If I call it directly

   firefox file:///home/tim/perl/foo.html

it works fine.

I don't seem to have a Settings → Files and Applications → Applications menu (Xfce 4.18).

I changed the "text/html" setting in Applications -> Settings -> Default Applications to Firefox and now it works as desired. The question is, why wasn't having the default Web Browser set to Firefox enough? Why does the MIME type also have to be changed?

The chosen HTML handler is expected to take priority over the default browser when opening file:///example.html. On Ubuntu 23.10 with GNOME, changing the default browser causes the HTML handler to reset back to default.

Oh, excellent! Thanks so much!