Starting from Firefox 84.0 - weather's details are hidden. On updated version 84.0.2 it's still the same:
Don't know who should fix it, maybe Firefox, or maybe CSS can be done better.
One workaround for now is to unset overflow: hidden.
%USERPROFILE%\AppData\Roaming\Mozilla\Firefox\Profiles\[PROFILE-CODE].default\
Open or create folder: chrome
Open or create file: userContent.css
@-moz-document domain(garmin.com) { #weather-controls > .map-control-item { overflow: unset !important; } }