Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

If there is still a problem, Start Firefox in Safe Mode {web link}
by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh).
Did this help?

While you are in safe mode;

Try disabling graphics hardware acceleration in Firefox. Since this feature was added to Firefox it has gradually improved but there are still a few glitches.

How to disable Hardware Acceleration {web link}

Hi rwill1, little changes in how a form field appears when it has the "focus" may be difficult to search up. As a starting point, do you recall when this started? Firefox 78.0.1 was just released in the past few days while Firefox 77 was released 4-5 weeks ago.

After the last update.

I noticed this in the release notes for Firefox 78.0:

"Text input controls with custom styling now correctly show the focus outline when appropriate."

I don't think there is a setting to turn that off since it was to make a feature work as designed that wasn't working as designed.

Hopefully the Sudoku site can tweak its rules to clean that up.

For what it's worth, the focus ring on Windows 10 (attached) is very subtle compared with Mac. What does Safari look like?