I have a few guesses:

Google has marginal incentive to not kill Firefox (antitrust), they have no incentive to make sure they provide a good experience on Firefox, let alone test with it.

The issue seems to be isolated to the search home page, and Google Search hasn't exactly been associated with "quality" in several years. Internal rot and disinterest gradually chip away at QA.

They don't test on Firefox mobile.

Not explicitly anyway. Google's testing strategy is as it has always been: do some in-house burn-in testing, then launch to a small number of users and check for unexpected failures, then widen the launch window, then launch to 100%.

In this case, the user base in question is so miniscule that no bugs probably showed up at the 1%, 10%, and 50% launch gates.

Testing can never prove something works, only that something doesn't.

They test on version 64 ;)

simply: they don't test

at least not for firefox mobile

they probably test for chrome (desktop+mobile), safari (desktop+mobile), edge(desktop) and maybe Firefox (desktop) but probably no other browser

Test written according to behaviour?