Rated 5 out of 5 stars

You have an extra closing tag in the firesizer.xul file.

Either get rid of the <"/>" or <"/menupopup>" after the opening <"menupopup>" tag.

This review is for a previous version of the add-on (1.6.1-signed.1-signed). 

Rated 5 out of 5 stars

- Extract the .xpi using something like 7-zip
- Open 'install.rdf' and change maxVersion under <!-- Firefox --> from 6.* to 7.*
- Save the file then add back to the .xpi
- Install add-on from file
- Rejoice!

This review is for a previous version of the add-on (1.4.1-signed.1-signed).