Finally. As of the latest update the Firefox Beta for Android (v28.0b8) is usable on my low cost Huawei Fusion 2. Thanks Mozilla developers!
I’m currently running the 5 March 2014 release, which is the first one that has really been usable for me. Previous versions had a bug that messed up rendering when the screen was switched from portrait to landscape or the soft keyboard popped up. In the former case the display would become completely unreadable, in the latter important screen elements like the entry window were blacked out.
It also appears that previous problems with CPU and memory utilization have cleared up as well, although that may be because of fixes to the more fundamental video display issues.