Revert an addition of contain: strict

I thought that adding isolation: isolate to the React root had fixed the Firefox layering glitches, but today I've started noticing those glitches again.

src/room/InCallView.module.css

@@ -28,7 +28,9 @@ limitations under the License.
   flex-direction: column;
   overflow: auto;
   overflow-inline: hidden;
-  contain: strict;
+  /* There used to be a contain: strict here, but due to some bugs in Firefox,
+  this was causing the Z-ordering of modals to glitch out. It can be added back
+  if those issues appear to be resolved. */
 }
 
 .centerMessage {