Why has compose:rwin stopped working in Firefox and Thunderbird?

[Adam Funk]

For a long time I've had the line

    Option  "XkbOptions"  "compose:rwin"

in the InputDevice stanza of my xorg.conf file.  Recently (since I
upgraded from Feisty to Gutsy, I think) I've noticed that the compose
key still works in other applications but is ignored in Firefox and
Thunderbird.  What's up, and how can I correct this?

Thanks,
Adam