Apparently this is due to an unrelated change in the Tree Style Tabs plugin, and is not the fault of Firefox itself in the least: https://github.com/piroor/treestyletab/issues/3575
Just noticed this myself within minutes of updating, and now editing things in textareas is basically unusable for me. I've been frantically searching for settings to undo this. (FWIW I typically use the common Cmd-Shift-[ and Cmd-Shift-] shortcuts t...
