SOLVED - Problem caused by missing one asterisk (*) in a section title whilst updating MrOtherGuy's Compact UE panel CSS to Fx 115... finger trouble/PEBCAK. Apologies, and many thanks for all the help. :)
Updating from Firefox 114 > 115 the Unified Extensions panel has notably increased in size by approx 30% even when using MrOtherGuy's updated 'compact_extensions_panel: make it work in Fx115' CSS: https://github.com/...
Additionally, attempting to reduce the icon size or line padding makes no difference whatsoever.
Has some underlying code changed in Firefox 115's UI?
(The eagle-eyed may notice the RH image is actually from the not yet updated LibreWolf 114.0.2-1)
They said a while back that various UI paddings were getting a little plumped to make the UI easier on mobile devices. It's just life, I'm afraid.
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