Compare the two Firefox windows on the screen to the left, with the same windows (a different Firefox profile) on the screen to the right:

What might be wrong?
My Firefox font preferences (below) are not unusual.

Coincidental but maybe non-related: the Bookmarks menu of KCharSelect repeatedly loses the ability to display some of my bookmarked characters.
A shot from 7th March:

When I prefer a different font, there's an almost immediate fix. After quitting then reopening KCharSelect: where previously there were blank/empty spaces, the characters appear.
Unfortunately, the fix is not long-lasting. Sooner or later (maybe with the next Plasma session), KCharSelect has the same Bookmarks menu misbehaviour with the font that previously seemed to provide a fix.
Very shortly after a switch to Noto Sans 9 pt:

– OK so far … OK following a log out … OK following a stop of the computer.
Maybe there's an intermittent problem (not immediately apparent) with fonts such as Noto Mono Medium.

What might be wrong?
My Firefox font preferences (below) are not unusual.

Coincidental but maybe non-related: the Bookmarks menu of KCharSelect repeatedly loses the ability to display some of my bookmarked characters.
A shot from 7th March:

When I prefer a different font, there's an almost immediate fix. After quitting then reopening KCharSelect: where previously there were blank/empty spaces, the characters appear.
Unfortunately, the fix is not long-lasting. Sooner or later (maybe with the next Plasma session), KCharSelect has the same Bookmarks menu misbehaviour with the font that previously seemed to provide a fix.
Very shortly after a switch to Noto Sans 9 pt:

– OK so far … OK following a log out … OK following a stop of the computer.
Maybe there's an intermittent problem (not immediately apparent) with fonts such as Noto Mono Medium.