When playing html5 videos with my 1+1 (Cyanogenmod 11 – snapshot M11 – Android 4.4.4 with ART runtime) using Firefox 33.1 (version 34 does not fix the bug either) audio works fine while video is completely corrupted with grey artifacts all over the place.
The problem appears to be quite common and is not only circumscribed to 1+1.
The best workaround so far is type about:config in the address bar, search for media.stagefright.omxcodec.flags and set its value to 8 to disable video hardware acceleration (0 lets android pick the best option and 16 forces hardware acceleration always on).
Firefox 36, which is currently in nightly stage, should come with a patch that will actually solve the issue but I am not comfortable with running a browser in alpha/beta stage so for now I will live without video hardware acceleration.

.:. Update

Firefox 36 solved the issue, now hardware acceleration works like a charm.