I read that others are reporting PWM to still be broken, which gave me a clue: I had recently installed the Noia theme. That is what changed, not Firefox itself. Having realized that it was I who had made a change to my Firefox setup, I thereupon changed back to the Firefox default theme and tested PWM, and, yes, with that theme, PWM is still broken.
Just to be certain, I reverted to Noia and tested PWM, which then worked properly.
Perhaps this discovery will help the PWM programmers to figure out what is amiss.