FF139 Relnote - SVG discard removed (#39637)

* FF139 Relnote - SVG discard removed

* Update files/en-us/mozilla/firefox/releases/139/index.md

Co-authored-by: Brian Smith <brian@smith.berlin>

* Update files/en-us/mozilla/firefox/releases/139/index.md

Co-authored-by: Brian Smith <brian@smith.berlin>

* Update files/en-us/mozilla/firefox/releases/139/index.md

---------

Co-authored-by: Brian Smith <brian@smith.berlin>

Author: hamishwillee

files/en-us/mozilla/firefox/releases/139/index.md

@@ -50,6 +50,10 @@ This article provides information about the changes in Firefox 139 that affect d
 
 #### Removals
 
+- The experimental `<discard>` element has been removed, along with its corresponding {{domxref("SVGDiscardElement")}} JavaScript interface.
+  These aren't supported in other browsers and are expected to be removed from the specification.
+  ([Firefox bug 1958839](https://bugzil.la/1958839)).
+
 ### HTTP
 
 #### Removals