I couldn't play my move and I lost the game

Yeah, this never happens that's why it's so weird. Pinging works fine for both 8.8.8.8 and lichess.org. The tab is always open and in focus, and my PC is fine. The only thing it could be is either my adblocker (which I have turned off for Lichess), DuckDucKGo Privacy Essentials extension or the Firefox's built-in HTTPS only feature (but I can't really see that as being an issue as Lichess has support for https). But, yeah this never happens it's a weird one-off.

Maybe it is related to Lichess as you say.

But I would say it is common to get disconnected several times a day, but you usually do not pay attention on normal Internet surfing. During a chess game it is more noticeable, as there is a live interaction.