Reproducible on Firefox OS Simulator 4.0 and also on the latest preview versions. Tested with both Firefox Aurora 25.0a2 (2013-08-20) and Firefox Nightly 26.0a1 (2013-08-17). Spotted on Microsoft Windows 7 x86-64 but I have also reproduced it on an up-to-date Arch Linux with KDE.
Steps to reproduce: open Firefox OS Simulator and open the Browser app. Inside the browser, navigate to http://leafletjs.com/examples/geojson.html and try clicking on any of the features on the map (circles or polylines).
Intended behavior: after clicking (emulated tapping) the features, popups should open with example text.
Real behavior: the map view focuses as if the popups were open but nothing appears. Only after a little not-so-straightforward clicking effort (double-clicking seems to sometimes solve the issue) a popup will open.