I have noticed on a few sites that the Style Editor does not display the whole stylesheet. As soon as your start editing, the styles get messed up.
is an example. The inove/styles.css file (which I believe is the main theme style for this WordPress site) gets chopped off.
This is using version 0.5.2 of the xpi (I don't know if this bug exists in the current patch) running on Nightly 2011-11-02.
It is in the patch as well (no feature difference on loading).
Investigated this and bisected it to happen after 0.3 due do asynchronous stream conversion change for pages whose style sheets are served with Content-Encoding: gzip).
This is fixed in add-on v0.5.4 and the next patch update on bug 583041 :
Browser patch on bug 583041 updated with this fix.