Maybe extension developers haven't yet modified extension to work under Firefox 1.0. You can try following:
Open .xpi file with Ark, or some other archiving utility, and extract file install.rdf. Edit the file with some text editing program like this:
Code:
<em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id>
<em:minVersion>0.7</em:minVersion>
<em:maxVersion>1.0</em:maxVersion>
Notice
maxVersion.
Put the file back in the XPI archive, and try to install extension again.