Volume Boosting by Browser

For playing a file you have loaded yourself, the four major browsers are equivalent: they all implement the Web Audio API and will process the same graph identically. Differences in perceived loudness between them almost always trace back to the operating system holding a separate level for each application rather than to anything the browser itself is doing.

The meaningful split is over capture. Chromium-based browsers let a page request a tab and receive its sound alongside the video; Firefox and Safari implement screen sharing without offering that track at all. This is a deliberate difference in approach rather than an oversight, and no flag or preference changes it.

Letting a web page listen to another tab is a serious privacy concern, so the capability is gated behind an explicit user gesture: you must actively pick a tab from a browser-drawn dialog and tick a box to share its audio. No page can start capturing on its own.

Firefox and Safari have not implemented audio capture in their screen-sharing APIs at all. That is a deliberate difference in approach, not a bug, and there is no flag or setting to enable it.