If you're running WinXP then you don't need WinZip - just rename the .xpi file to .zip, make the changes, then rename it back to .xpi. Probably worth keeping a copy of the original in case anything goes amiss.
If you've already tried to install this extension / addon then you will also need to modify "
... such that the values for minVersion and maxVersion match those in the "install.rdf" file within the .xpi file / package.
Once you're done, place the .xpi file into "