Since version 89, the free and open-source Firefox web browser makes use of the new ‘Proton’ UI design.
For those who do not like the new Firefox appearance, the configuration options to re-enable the old style interface is still available in the 89 release.
Revert to old UI Design in Firefox 89:
about:config in the address bar and hit Enter. Click on the ‘Accept the Risk and Continue‘ button when it prompts.
Next type browser.proton in the search box. Then click on the double arrow icons to disable the values of:
The Firefix UI will change immediately after the settings. And you can reset these settings to get back the new ‘Proton’ UI design.
In addition, the previous settings is going to be removed in next release according to this bug. So using a browser theme could be a good choice for long time solution.