I searched for some solutions and came up with two that were pretty slick. I really would like a standalone lightweight XPS viewer, but they don’t seem to exist.

Solution 1: Use the IETab extension

Instructions from http://channel9.msdn.com/ShowPost.aspx?PostID=259840
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Install the IETab extension from http://ietab.mozdev.org/

Add a filter for xps extensions or modify the mht filter to read:

/^file:\/\/\/.*\.(mht|mhtml|xps)$/

Presto – XPS docs open in an IE tab within Firefox.

Solution 2: Associate the .XPS extension with Internet Explorer

Hold down shift, right-click on an XPS file and choose “Open With…”. The default should be “XPS Viewer”. Select Internet Explorer and then check the box “Always use the selected program to open this kind of file”.

For some reason this doesn’t seem as elegant as solution 1, but it gets the job done without having to install any additional plug-ins.