No Longer Able to Login to fitbit.com via Firefox Browser Specifically

[pgtr]

I've had this problem for a year or so. I've searched various threads and google to find a possible cause or solution. No joy. I can login to my fitbit account thru other browsers but NOT thru Firefox. I enter my credentials, select the 'Sign In' button and the Fitbit logo pops up and 'pulses' indefinitely as if logging in... And that's it. 

If it helps any - it doesn't matter if the credentials are valid or not. I get the same behavior with Firefox. E.g. I can enter made up nonsensical credentials - it just displays the pulsing Fitbit logo indefinitely after selecting sign in.

Currently Firefox 125 (updated regularly) (64 bit) and Windows 10

I've downgraded Firefox security to "Standard"

I've Cleared cache and cookies and history many many many times

I've added fitbit.com as an exception to allow all cookies

I've added fitbit.com as an exception to pop-up windows

I've added fitbit.com as an exception to install add-ons

I am not seeing any further settings in Firefox to make it any 'looser' or less secure for the sake of trying to login to fitbit.com.

So for this one lone website - I cannot use my browser of choice and must install and maintain a non-Firefox browser. But it would be nice to find a better workaround or solution to not being able to login to fitbit.com from a popular and commonly used browser like Firefox.

Suggestions?

[JohnnyRow]

No trouble here with Firefox on PC Windows 10.