"works" in quotes. Compact after an update I measured as a third taller than "normal" density before the update (100px vs 75px). I want maximum vertical space for the website, not for the browser UI. It's really not compact anymore.
The searchbar looks odd now. Something weird going on with the font rendering as well (on Linux) ... ugh.
Looks like they made it harder to find the option to customize the toolbar.
Right click on toolbar, select 'Customize Toolbar', click 'Density' option at the bottom (I'm macos).
Looks like "Compact" is only in the list, if you had it selected prior. If not, then go to about:config and set "browser.proton.enabled" to false. Then, you'll get the option.
What a mess.
You're looking for the "browser.compactmode.show" key.