Hard to say anything without an example. Do you have a URL of where
this happens?
As far as I know there is no problem with the clock rendering in
Firefox, and well I use Firefox as my default browser.
I know that Google Ads also uses frames (like the clock does), there
maybe a situation where frames are being mixed up, maybe because you
didn't fully reload your page when you uploaded it. Try reloading with
ctrl-R. Other than that, I don't know what else to suggest, without a
concrete example to analyze.
Take care,
Otto