Late update: I feel extremely stupid. I'm using an app called "App Tamer"[0] and had it configured to run Firefox on the efficiency cores only, even when it's in the foreground. Maybe I thought originally that the e-cores would be more than fast enough to handle a browser, but I guess that's not the case. This also explains why I might not have noticed this issue before: I only use Firefox for work-related tasks and I rarely work while on battery.

[0]: https://www.stclairsoft.com/AppTamer/