Exactly the same error message ... both on regular gmail and the one using Google Apps.
Seems like there's no classes on the compose page by the name "goog-imageless-button-content" nor "J-K-I-Jz" and no error handling for this case.
I tried out some stuff and nothing I did worked so I think it isn't related to the following:
- languages (tried swedish and english)
- plain/RTF made no difference
- theme (changed to default)
Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.3) Gecko/20100409 Ubuntu/10.04 (lucid) Firefox/3.6.3
Mail me if you want me to check something specific ...