In Firefox 91, restore the old tab style as follows:

1. Open about:config.

2. Search for toolkit.legacyUserProfileCustomizations.stylesheets.

3. Double-click the value to set it to true.

4. Open about:support.

5. Search for Profile Directory (or Profile Folder).

6. Click Open Directory (or Open Folder).

7. Create a directory named chrome.

8. Open the chrome directory.

9. Create a new file inside chrome named userChrome.css.

10. Copy and paste the following code into userChrome.css:

     .tab-background {
       border-radius: 0px 0px !important;
       margin-bottom: 0px !important;
     }
    
     .tabbrowser-tab:not([selected=true]):not([multiselected=true]) .tab-background {
       background-color: color-mix(in srgb, currentColor 5%, transparent);
     }
    
     menupopup>menu,
     menupopup>menuitem {
       padding-block: 2px !important;
     }
    
     :root {
       --arrowpanel-menuitem-padding: 2px !important;
     }
    

11. Save the file.

12. Restart Firefox.

The old tab style is restored.

In Firefox 91, restore the old tab style as follows:

1. Open about:config.

2. Search for toolkit.legacyUserProfileCustomizations.stylesheets.

3. Double-click the value to set it to true.

4. Open about:support.

5. Search for Profile Directory (or Profile Folder).

6. Click Open Directory (or Open Folder).

7. Create a directory named chrome.

8. Navigate into the chrome directory.

9. Create a new file inside chrome named userChrome.css.

10. Copy and paste the following code into userChrome.css:

     .tab-background {
       border-radius: 0px 0px !important;
       margin-bottom: 0px !important;
     }
    
     .tabbrowser-tab:not([selected=true]):not([multiselected=true]) .tab-background {
       background-color: color-mix(in srgb, currentColor 5%, transparent);
     }
    
     menupopup>menu,
     menupopup>menuitem {
       padding-block: 2px !important;
     }
    
     :root {
       --arrowpanel-menuitem-padding: 2px !important;
     }
    

11. Save the file.

12. Restart Firefox.

The old tab style is restored.