There is an unnatural color stroke on the bottom half of toolbarbutton and tab text when a persona is applied, which makes it look fuzzy. For the tabbar, I think is because of a rule in browser.css that applies a text-shadow to tab text: http://mxr.mozilla.org/mozilla1.9.1/source/browser/themes/pinstripe/browser/browser.css#1548 Toolbarbutton text also has a text-shadow, but I can't find the rule that applies it. In any case, we should be able to remove them from both with rules for .toolbarbutton-text and .tab-text that remove the text-shadow.
Fixed for both Firefox 3.0 and 3.1 (which required different fixes) in changeset 8af7398d2daf. Fix has been incorporated in latest dev build: http://people.mozilla.com/~myk/personas/dist/personas-dev.xpi