42252 add new relative units ff release (#42470)

* added firefox release note for root font based units

* Update files/en-us/mozilla/firefox/releases/147/index.md

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

---------

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

Author: dletorey

files/en-us/mozilla/firefox/releases/147/index.md

@@ -50,6 +50,8 @@ Firefox 147 is the current [Beta version of Firefox](https://www.firefox.com/en-
   ([Firefox bug 2001878](https://bugzil.la/2001878)).
 - The {{cssxref("counter-increment")}}, {{cssxref("counter-reset")}}, {{cssxref("counter-set")}}, and {{cssxref("quotes")}} properties are now supported on the {{cssxref("::marker")}} pseudo-element.
   ([Firefox bug 2000404](https://bugzil.la/2000404)).
+- The following [relative length units based on root element's font](/en-US/docs/Web/CSS/Reference/Values/length#relative_length_units_based_on_root_elements_font) are now supported: `rcap`, `rch`, `rex`, and `ric`. These units allow you to define `<length>` values based on the size of a particular character or font attribute of the [root](/en-US/docs/Web/CSS/Reference/Selectors/:root) element.
+  ([Firefox bug 1740584](https://bugzil.la/1740584)).
 
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