Consider adding options to revert the look back to Firefox 88 (Photon).

[yum13241]

In my opinion the modern Proton UI wastes space, when you click on the hamburger in modern Firefox on a 1366x768 (low res IK), you can't fit the entire menu (ARGH), but you could on older versions, before this stupid Proton UI came to Firefox, and unfortunately to Mercury.

So you could show the compact mode switch by default. When its enabled, it obviously makes things more compact, but the tab bar still looks stupid. To remedy this, you can use Firefox-UI-Fix (Photon Style). It'd be great if there was button for it in the options menu, similar to how Floorp does this. I know this is compiler-optimized Firefox not Floorp, but it seems like you also like de-crapifying browsers (Replacing Cr23 in Thorium is a good example).