In line 216 of ./browser/base/content/credits.xhtml , there is a space between "&credit.thanks2;" and the link "&credit.contributors2;". Since not all language separates words in space, it's logical to include the space in DTD entires, e.g., &credit.thanks3; instead of hard-coded it in xhtml. (this is a bug from external QA)
Credits.xhtml was removed by bug 579547. It's replacement now includes the space in the localisation entry, so translators can correct as needed: http://mxr.mozilla.org/mozilla-central/source/toolkit/locales/en-US/chrome/global/about.dtd#3 -> Fixed.