I think it has to do with the operating system on your machine. On my home PC (using Windows 10 Home), I have the same issue regardless of browser. At work, using a business version of Windows 10, it loads all frames up to the 40 max regardless of browser. Beats me how to resolve it. With WU now, I'm not holding my breath for any fix, much less one soon.
[edit]...low and behold, it looks like they changed the url for the radar to get the full 40 frames. Try this: go to the main header and under 'Maps & Radar', choose 'Radar Maps'. Then click on the area of the map you're interested in. The map should zoom in to that area and have radar icons where stations are. Click the icon you're interested in and try that. If it works, see if the url is different from the one you've been trying. Good luck, it seemed to work for me.
[edit #2]: doesn't seem to work for high definition radar though...
[edit #3] (sorry for all the edits): it looks like maybe it's only fixed for some radars. The one I use most seems to be fixed (other than high def) but I randomly chose some other radars nearby and they still seem to be stuck on 12 frames max. Beats me what's going on.