The (First World) Problem

Firefox 89 brought significant changes to the look and feel when it was pushed out in early June, 2021. Seems like people either didn't care or absolutely hated the change. If you were pressed for screen real estate, e.g. on older/smaller laptops, you tended to fall into the second camp.

The Solution

Here's what worked for me:

1. go into about:config, search for the setting browser.compactmode.show and set it to "true". Then right-click on the titlebar, select "customize", and set the "density" to "compact" in the toolbar that appears at the bottom of the screen.
2. install the Photon Colors theme from the Firefox themes gallery or this location https://addons.mozilla.org/en-US/firefox/addon/photon-colors/
3. finally, go to this Github page https://github.com/black7375/Firefox-UI-Fix#installation-guide and follow the instructions to install the Lepton user chrome package.

The Lepton package really tightens down the layout so both the tab bar as well as the hamburger menu consume only a little more than half the vertical space of the default layout. It will also get rid of the "floating" tabs and reattach them, to where they belong.

With these tweaks FF89 looks a lot like FF88 did, and the layout is even a bit more compact.