Nope, just installed FF28 on Win7 and I don't get the problem here either. If it is tvheadend, it's a specific problem with your installation.
I'm on 3.9.438~gd328f74 - what version are you using? We can dig further if needed...
EDIT
You said what version. D'oh. Maybe try something else... live with it until FF29 if you can, and then shift to 3.5 or 3.9?