just hack adblock
I've got adblock 0.5.3.043 running fine on 3.0b5.
You work on the .xpi extension file:
1. rename the .xpi file to .zip
2. open the .zip file and edit the install.rdf file, with the FireFox section edited to look like:
<!-- FireFox -->
<em:targetApplication>
<Description>
<em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id>
<em:minVersion>0.7</em:minVersion>
<em:maxVersion>3.0.*</em:maxVersion>
</Description>
</em:targetApplication>
3.Save install.rdf and close the .zip file
4. rename the .zip back to adblock-0.5.3.043.xpi
5. Go to FireFox 3 and do File | Open file to locate the edited file and install adblock.