One option is to attempt (despite of all of the obstacles they might put in place..) to increase Firefox's market share, perhaps by spreading the word about privacy concerns and/or ux benefits.

Another might be adblocker-esque trusted and shared community-maintained lists along with an add-on (does this exist?) for per-domain user agent mocking. Hopefully with enough adoption (i.e integrated into ublock) that it reduces the value proposition of investing the time.

Thirdly, continuing to call out the companies who do this sort of thing.