Gecko could always be updated, but it's a really non-trivial job so no one ever did it. Microb doesn't use XUL.
$ find / -name *xul* 2>/dev/null
/usr/lib/microb-engine/libxul.so ... /usr/share/microb-engine/defaults/pref/xulrunner.js ... /usr/share/microb-engine/chrome/browser/content/navigator/navigator.xul /usr/share/microb-engine/chrome/browser/content/browser/browser.xul ...