Is Preferences->General->Zoom->Default zoom what you are looking for?

Aha! It's that simple! Thanks. You should post that as an answer. "Default zoom" is just "Skalering" in my language, though, but I guess covering translations in user interfaces is outside the scope here. That said, we do need a discussion about it elsewhere (not this comment section), because I frequently struggle to follow answers because the language is different, some names seriously deviate, and because the system often won't let me search for the English ones. Anyway.

Two problems with this approach, though:

1. It will instantly rescale every webpage, making you lose existing settings, which is unfortunate. I don't want every webpage at 80%, just most of them. Some I keep at 100%, while others at 90%.
2. It provides far less options than when scrolling with the mouse wheel. For instance, the mouse wheel lets you set 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, 110%, etc, while in the settings, you can only choose 30%, 50%, 67%, 80%, 90%, 100%, 110%, 120%, 133%, etc.

Easy solution to the language problem: use the english version of firefox :P

(maybe make a meta question for a more helpful answer how to treat such problems)

Maybe you can adjust the available zoom levels in about:config->toolkit.zoomManager.zoomValues