Rated 5 out of 5 stars

1. Modify xpi file:
- Rename bordercolors-0.6-tb.xpi to bordercolors-0.6-tb.zip
- Extract install.rdf from bordercolors-0.6-tb.zip
- Edit number behind <em:maxVersion> in install.rdf
- Add install.rdf to bordercolors-0.6-tb.zip
- Rename bordercolors-0.6-tb.zip to bordercolors-0.6-tb.xpi
2. Add-ons-Manager of Thunderbird:
- Remove deactivated BorderColors
- Install modified BorderColors:
- gear wheel Icon -> Install Addon from File...
- Browse to bordercolors-0.6-tb.xpi
- Install.
3. It works with Thunderbird 7.0.1
4. It even remembers the color settings, which were last used under Thunderbird 3

This user has 2 previous reviews of this add-on.