That tone-deaf non-apology was only on reddit, by a new-on-the-job “community specialist”. They did a (slightly) better apology an hour later on twitter and the GOG forum: https://www.gog.com/upload/forum/2026/06/c1cdf9fda39aaa5532ee375b38abb04376c7505d.png
GOG works absolutely fine on Firefox for me. You might have some broken script stored in cache or somesuch. Try a hard reload (ctrl-shift-R).
10 years ago they intended to, then dropped it. Starting this year, they looked at it with renewed interest, hired a senior dev for it, and have actually been working on it.
Bizarre! Any extensions installed which might block script loading? Try in a private window, and if that works, clearing cookies. I haven’t seen anyone on the GOG forums report anything like this, while at least half the people there use Firefox, so it should be fixable on your side somehow.