Cannot insert cells in Sage 4.7 with ubuntu 11.04

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Hi,

We recently found that our SAGE 4.7 in Ubuntu 11.04 has the following problem with the notebook. We use firefox 6 and everything works fine except that we cannot insert cells, even if the blue line apperas. If we press ALT+ENTER, then we can evaluate the input and produce a new cell, but we would like to be able to insert cells as usual.

Thanks in advance,

Joaquim