Custom Default Preset: Firefox 131+ with Fixed-Width Multirow-Tabs

[macrogreg]

This is the a nice configuration based on the Custom CSS for Firefox mod by Aris-t2.
Use it as-is, or as a basis for your own modifications!

Features:

• Multi-row Tabs with fixed width.
• Close-tab buttons appear only when hovering the mouse above tabs: Avoid hiding tab titles.
• Playing sound icon always visible: Easily find/mute/unmute playing tabs.
• Classic buttons.
• Firefox 133 support.

[macrogreg]

My apologies. I intended to create this PR against my own fork, but selected the master repo by mistake.
Closing this PR in favor of the identical PR within the fork, which is merging my mod into my fork.