How many tabs do you have open? I have quite a few and it can cause battery to drop a lot. It also occasionally starts using a lot of battery in the background for no apparent reason, which is fixed by just force-quitting the app.
It could also just be that Firefox is inherently more energy intensive than a Lemmy client. Browsers have to handle a lot of stuff to render web pages, while clients for sites just have to send some API calls. I might try using another browser for a few days and see if the battery usage is comparable to Firefox.