Ok, so you're having trouble installing the plugin. This is easily
fixable, but you have to mess up with the .xpi.
Here's the step-by-step:
1 - download the xpi from the lightning web page (save it, but don't
install it just yet)
2 - open a terminal and go to the folder where the xpi was saved.
3 - create a temp folder and enter it (mkdir lightning-tmp && cd
lightning-tmp)
4 - unzip ../lightning-0.7-tb-linux.xpi (adjust to the correct file name)
5 - open install.rdf with you favorite editor
6 - find the tag <em:targetPlatform>
7 - change the value of this tag to linux-gnu_x86-gcc3 (e.g.
'<em:targetPlatform>linux-gnu_x86-gcc3</em:targetPlatform>')
8 - save the file and exit the editor
9 - re-zip it (zip -r ../lightning-0.7-debian.xpi)