Question: I am running Firefox 90.0 on Ubuntu Mate 18.04 32-bit. It seems that Firefox'es tabs and menu bar are optimized for smartphones as they are twice as high as the bars of all other programs. My netbook has a small screen and so I'd like to reduce them to ordinary size. How/is that possible?

Answer: 
**OPTION ONE:**

First, go to `about:config` in your URL bar.

Then, search for `browser.compactmode.show` and change the value to `true` .

Finally, go to the **Hamburger-Menu** > **More-tools** > **Customize-toolbar** and at the bottom of the page, click **Density** and select **Compact**.

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**OPTION TWO (recommended):**

First, go to `about:config` in your URL bar.

Then, search for `layout.css.devPixelsPerPx`

Finally, double click on the value and change it to less than `1.0`. After you set the value, press `ENTER` to apply the changes. For example, you can use `0.75` or if that is too big, you could try `0.5`.
If you are referring to the header bar in Firefox v90 then there are facilities to remove it.

* Click on the 3 lines (so called hamburger) icon to open Firefox menu.
* Click `More Tools` then `"Customise …`” menu option to open new Customise Firefox
tab.
* Finally un-check the “Title Bar” check-box in the bottom-left.

[reference](https://ubuntuhandbook.org/index.php/2020/07/remove-header-bar-firefox-ubuntu-20-04/)