Do you have the latest version?
https://addons.mozilla.org/en-US/firefox/addon/default-fullzoom-level/

Version 4.5 March 4, 2011 34.8 KB

Works with:

   * Firefox 3.6 - 8.*

I have Firefox 6.0

The interesting, I have tried this addon in this month (full zoom level or zoom level I not remember exactly), and it is worked in Firefox 5.0, now it is disabled in firefox 5.0 and 6.0, because it is not compatible...

Delete the files extensions.* (extensions.sqlite, extensions.ini, extensions.cache, extensions.rdf) and compatibility.ini in the Firefox profile folder to reset the extensions registry.
New files will be created when required.

See "Corrupt extension files":

• http://kb.mozillazine.org/Unable_to_install_themes_or_extensions

If you see disabled, not compatible, extensions in "Tools > Add-ons > Extensions" then click the Tools button at the left side of the Search Bar (or click the "Find Updates" button in older Firefox versions) to do a compatibility check or see if there is an update.

This is a new installation of Firefox. The previous (5.0) was on an other computer. It means Default Full Zoom Level incompatible with Firefox 6.0?

Did you do a compatibility check?

That is required in cases where the extension has a maxVersion (in this case 4.0.*) set in the XPI file and the server has the correct compatibility data ([8.*]).

From https://addons.mozilla.org/en-US/firefox/addon/default-fullzoom-level/
Works with: Firefox 3.6 - 8.*

How can I do a compatibility check?