I didn't understand what you meant first, clicking on my back arrow on the physical mouse, or the go back icon in the browser and Firefox would go back like expected. Then I tried Alt+Left arrow on the keyword, and it didn't work! But it wasn't just not going back, like when you have a "too many history.pushState()" bug that fucks it up for you. Instead it went to the bottom of the page. Then I noticed it was selecting some block in the bottom right and that it also happens when you just press left arrow on the keyboard without Alt.

So TLDR: bug with some selection/focus thing on the Notion page. Poor execution, I rate their implementation of "static notebook HTML pages" 7/10.