This archive contains the file userChrome.css which changes the tabs design in Firefox 89+ (Proton UI) to design similar to the one from Firefox 88.

Firefox 89+ tabs look (with default Dark Theme <link>, ProtonUI disabled (steps 1-3) and compact mode set (steps 4-6)):

The tabs after using userChrome.css from this repo:

Tested in Firefox 91.0.2 and 92.0.1

Usage

1. Open Firefox and type about:config into the address bar and hit enter
2. In the search box, enter proton
3. Set the following settings to false:

   browser.aboutwelcome.protonDesign
   browser.proton.enabled
   browser.proton.places-tooltip.enabled
   

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4. Find and set the following setting to true:

   browser.compactmode.show
   

5. Click on menu symbol ≡ -> More tools -> Customise toolbar...
6. At the bottom, click on Density and choose Compact

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7. Find and set the following setting to true:

   toolkit.legacyUserProfileCustomizations.stylesheets
   

8. Open new tab, enter about:support and hit enter
9. Search for Profile Folder and click Open Folder
10. In that directory create chrome folder
11. Inside, paste the file userChrome.css from this repository and restart Firefox