> Outside of media heavy applications such as video conferencing software, do websites explicitly do specific things to “support” Firefox beyond complying with the specs?

You'd be surprised. Apple Business Manager does not support Firefox, for example.

> uBlock filter lists can provide fallback shims that would be loaded in place of ad scripts to deal with this exact problem. The shim implements a neutered version of the original script so that all the surrounding (non-malicious) code can continue to run without errors.

My point was that ads were being accidentally blocked and that websites wouldn't get their ad revenue.