Do you mean on the combined Stop/Reload button ?

Can you attach a screenshot?

• https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem
• use a compressed image type like PNG or JPG to save the screenshot

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Here's a clipped screenshot of just the area I had the original question about. What I was wondering is what was the yellow dot - now it looks like a sun - to the right of the combined Stop/Reload button. No idea what it is.

Is there somewhere that the icons used are labelled and possibly described? I searched for that but was unsuccessful.

I have never seen that. It may belong to one of your extensions.

It looks like your theme's background image. Try changing to the built-in Light theme.

Aha! That was it. A star in the Hubble theme. Thanks!

I still need to find a list of what all the icons mean and are called. I would have thought it would be something obvious, easy to find, but I can't find it if it is there.

A green dot under a (pinned) tab usually means to notify that the tab has been updated in the background and new, unread, data is present.

If you hover the mouse cursor over buttons in the toolbar, a tooltip often appears with more information.