"Adapt to TB 115" page needs to mention -moz-image-region going away

[eyalroz]

Many groups TB buttons used to use a single image, with a -moz-image-region custom property to choose which rectangle within the image to use for the button. However - in TB 115, that property is no longer recognized for buttons (or at all?) - and no TB-bundled extensions (messenger, composer, calendar etc.) use it any more.

That should be discussed on the page, and an appropriate alternative offered.

(TBH, I don't know what that alternative should be. Does -moz-image-rect work?)