relnote(Fx145): Matroska .mkv now supported (mdn#41820)

* relnote(Fx145): Matroska .mkv now supported

* Update files/en-us/mozilla/firefox/releases/145/index.md

Co-authored-by: Dipika Bhattacharya <dipika@foss-community.org>

---------

Co-authored-by: Dipika Bhattacharya <dipika@foss-community.org>

Author: bsmth

files/en-us/mozilla/firefox/releases/145/index.md

@@ -78,6 +78,8 @@ Firefox 145 is the current [Beta version of Firefox](https://www.firefox.com/en-
 
 - {{domxref("RTCEncodedVideoFrame")}} and {{domxref("RTCEncodedAudioFrame")}} are now {{glossary("Serializable object", "serializable objects")}}, and {{domxref("RTCEncodedAudioFrame/RTCEncodedAudioFrame", "RTCEncodedAudioFrame()")}} and {{domxref("RTCEncodedVideoFrame/RTCEncodedVideoFrame", "RTCEncodedVideoFrame()")}} copy constructors are supported. These changes allow frames to be cloned, and to be shared between workers and the main thread. ([Firefox bug 1868223](https://bugzil.la/1868223) and [Firefox bug 1975032](https://bugzil.la/1975032)).
 
+- The [Matroska container](/en-US/docs/Web/Media/Guides/Formats/Containers) (`.mkv`) is now supported for the most commonly-used codecs: AVC, HEVC, VP8, VP9, AV1, AAC, Opus, and Vorbis. ([Firefox bug 1991752](https://bugzil.la/1991752)).
+
 <!-- #### Removals -->
 
 <!-- ### WebAssembly -->