Not actually sure what you mean by New Tab Plus sign icon.

I do know that in it can possibly take your transparency from the Theme you are running. Please change theme to test that please.

I do not see any Extensions listed as New Tab Plus please give more information. If it is a Extension then Firefox does not do support for a Developers theme. Contact them or go to their web site for the support page.

https://www.howtogeek.com/333110/how-to-customize-firefox-quantum-and-remove-the-white-space-around-the-title-bar/

https://www.howtogeek.com/333805/how-to-change-or-customize-firefoxs-new-tab-page/

Please let us know if this solved your issue or if need further assistance.

The plus sign (A) should have square background box behind it. So you can clearly see it (B). Check picture.

jorb said

The plus sign (A) should have square background box behind it. So you can clearly see it (B). Check picture.

Change your Desktop theme or the Firefox Theme you are using and see if that makes a difference as it looks like a theme is coming in from behind as per transparency.

Please let us know if this solved your issue or if need further assistance.

This Theme filled the transparency. Now the plus sign is black with a solid light blue background. The new Theme seems to have fixed the problem. Thanks.

This has been raised as https://bugzilla.mozilla.org/show_bug.cgi?id=1423646