Clicking the pencil icon or navigating directly to #/editor/0 opens the wikitext editor, which then immediately closes, e.g. http://en.m.wikipedia.beta.wmflabs.org/wiki/Claude_Monet#/editor/0.
The Chrome DevTools Console shows the following error:
Error: Property "onUpdateState" is not a function Error: Property "onUpdateState" is not a function at validateMethod (eval at <anonymous> (http://en.m.wikipedia.beta.wmflabs.org/w/load.php?debug=false&lang=en&modules=jquery%2Cmediawiki&only=scripts&skin=minerva&version=eHtnlG51:4:681), <anonymous>:8:245) at OoUiToolbar.oo.EventEmitter.on (eval at <anonymous> (http://en.m.wikipedia.beta.wmflabs.org/w/load.php?debug=false&lang=en&modules=jquery%2Cmediawiki&only=scripts&skin=minerva&version=eHtnlG51:4:681), <anonymous>:8:487) at OoUiToolbar.oo.EventEmitter.connect (eval at <anonymous> (http://en.m.wikipedia.beta.wmflabs.org/w/load.php?debug=false&lang=en&modules=jquery%2Cmediawiki&only=scripts&skin=minerva&version=eHtnlG51:4:681), <anonymous>:10:135) at EditVeTool.OoUiTool (http://en.m.wikipedia.beta.wmflabs.org/w/load.php?debug=false&lang=en&modules=oojs-ui&skin=minerva&version=d8515bc6cc9d:82:118) at new EditVeTool (eval at <anonymous> (http://en.m.wikipedia.beta.wmflabs.org/w/load.php?debug=false&lang=en&modules=jquery%2Cmediawiki&only=scripts&skin=minerva&version=eHtnlG51:4:681), <anonymous>:6:2034) at OoUiToolFactory.oo.Factory.create (eval at <anonymous> (http://en.m.wikipedia.beta.wmflabs.org/w/load.php?debug=false&lang=en&modules=jquery%2Cmediawiki&only=scripts&skin=minerva&version=eHtnlG51:4:681), <anonymous>:17:36) at OoUiListToolGroup.OO.ui.ToolGroup.populate (http://en.m.wikipedia.beta.wmflabs.org/w/load.php?debug=false&lang=en&modules=oojs-ui&skin=minerva&version=d8515bc6cc9d:91:605) at OoUiListToolGroup.OO.ui.ListToolGroup.populate (http://en.m.wikipedia.beta.wmflabs.org/w/load.php?debug=false&lang=en&modules=oojs-ui&skin=minerva&version=d8515bc6cc9d:134:157) at OoUiListToolGroup.OoUiToolGroup (http://en.m.wikipedia.beta.wmflabs.org/w/load.php?debug=false&lang=en&modules=oojs-ui&skin=minerva&version=d8515bc6cc9d:88:750) at OoUiListToolGroup.OoUiPopupToolGroup (http://en.m.wikipedia.beta.wmflabs.org/w/load.php?debug=false&lang=en&modules=oojs-ui&skin=minerva&version=d8515bc6cc9d:129:767)
This was picked up in linux-firefox-sauce#885.