Over a decade ago, it was also possible to start a new browser engine from scratch. Now since the web is so complex, it's virtually impossible to make one nowadays.
I don't think you can compare the web of a decade ago to the web of now.
> 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.
You have to realize Mozilla does not have the marketshare to do this. Most of the web relies on ads, and since uBlock Origin blocks them, there would be no incentive for those sites to support Firefox.
Even if you just blocked trackers (as does ETP Strict Mode), many ads get blocked because those ads bundle tracking code within them. This, again, would cause there to be little to no incentive for most website owners to support Firefox.
What Mozilla is currently doing makes sense. They are being lax on the standard setting so that websites can still make ad revenue and have an incentive to support the browser.
If Firefox had a lot more marketshare, it might have been possible that this could slide.
I don't think you can compare the web of a decade ago to the web of now.