OK - I tried Chrome from a Windows system - no issues.
I tried my Samsung phone browser - no issues.
I'll do some more testing, but it may be a very specific issue. Unfortunately I don't have Firefox here to use, but I do have it on a PC at home. I'll see what I can come up with and get back to you.
Do you use any different windows colors, or pretty much standard? I've seen odd desktop/window color schemes that will throw off certain windows. I suspect you're using standard, but just checking to be sure.
Also, are you using any different fonts? Or font sizes. Just trying to cover all the bases. Or possibly... have the window zoomed in beyond 100%?