I would guess it is a setting in Firefox clearly it is getting the page, the failure to bring the application to the front might be a Firefox setting, but it is just as likely some "feature" of your Linux distribution so your work flow is not interrupted by another application taking the focus.

I see nothing in either of the Firefox settings, or Thunderbird to prevent hyperlink in email from immediately opening in default browser, so it must be a setting that is in config. Someone please help. If someone thinks it is because of the Ubuntu 23.10 update, can you tell me how to change a setting to allow the hyperlink to switch to the open tab in the browser?

I found the problem. When I updated to Ubuntu 23.10, the system installed some programs so there were two instances installed. I had to go and uninstall the app from the App Store, and leave the Software version running, which I believe one is a Snap version and the other (I can't think of the name of the depository) Software Center. Viola! TB now opens in my browser when I click on a hyperlink from email. Does anyone understand why this happens sometimes, when there are two different version from different installation (i.e., the Software Center, or the Ubuntu Software links?