Is it possible this is site-specific? The only issue I've had with Firefox on my MacBook was leaving pinned tabs open on pages that dynamically refreshed. Gmail, for example, would eat up memory over time. So I killed that pinned tab and I haven't had issues since. I still have Discord pinned without issue.

On iPad...I dunno, Firefox on iPad is a hard sell without extension support so I haven't used it much. I've been trying Orion lately, since it has a built-in ad blocker and is otherwise very similar to Safari in terms of performance and functionality.

I only run Linux on desktop so I'm not sure about battery life there. Is Firefox actually blocking sleep? I think Steam Deck runs a version of KDE, so perhaps you can use the kde-inhibit command to list and control blocks.