I am using Ubuntu 26.04, which by default installs Firefox as a snap. The snap cannot access the serial ports properly: the serial port choice says "no serial ports available". Running the .tar.xz download of Firefox works fine and does show serial ports.

This is a known issue: https://bugzilla.mozilla.org/show_bug.cgi?id=2038075. We should track this, and also document the problem in the Guides for now where we mention Firefox >=151can be used for Web Serial.