Another choice:

https://github.com/Merci-chao/userChrome.js#multi-tab-rows

Highlights

• Tab Groups Support: Fully supports mouse operations for tab groups — even in multi-row mode — delivering a smoother, more graceful experience.

• Enhanced Tab Animations: Adds fluid transitions for various tab-related actions.

• Optimized Space Usage: Makes full use of available UI space, including the area beneath window control buttons.

• Smooth Tab-Dragging Animation: Supports animated tab dragging even in multi-row mode.

• Pinned Tabs Grid Layout: Pinned tabs are fixed in a compact grid when Tabs Bar is scrollable — ideal for managing large numbers of pinned tabs.

• Native-Like Firefox Integration: Seamlessly aligns with Firefox’s behavior to support multi-row tabs as if natively built-in.

• Theme Compatibility: Fully compatible with themes, regardless of how many tab rows are present.