I had a quick look at the latest release description and it’s a little strange… In one place it says it’s based on Firefox 60.8:
SeaMonkey 2.53.18 uses the same backend as Firefox and contains the relevant Firefox 60.8 security fixes.
But then under ‘Known Issues’ it says:
The base user agent version was set to 91 to help with website compatibility
So… it seems like they’re just faking Firefox 91 in the user agent string. The actual featureset is from Firefox v60, which was originally released in 2018, and did not support aspect-ratio 
All of our officially supported browsers have had this aspect-ratio feature since 2021, so much of the Discourse UI now relies on it.