Quick Fix For Firefox 6 Rated 5 out of 5 stars

Download: http://www.sofken.com/exif_viewer-1.65-tb+fx_fx5.xpi
Rename File to: exif_viewer-1.65-tb+fx_fx6.xpi
Follow instructions how to edit an xpi file here: http://kb.mozillazine.org/Editing_an_add-on_to_change_its_compatibility

Actual required change in the install.rdf file:

em:maxVersion="5.0.*
TO
em:maxVersion="6.0.*

Finally it should look like this:
2.0.0.*) -->
<RDF:Description RDF:about="rdf:#$p+bJJ3"
em:id="{ec8030f7-c20a-464f-9b0e-13a3a9e97384}"
em:minVersion="1.5"
em:maxVersion="6.0.*" /><!-- Firefox (was 3.5.* or 3.6.0) -->

Tip:
Editing the file and saving can be done using an Archive Editor, then simply drop the modified file inside Firefox 6.
Job done. Enjoy :)

This review is for a previous version of the add-on (1.65).