browser.ctrlTab.previews has two separate functions:
Enabling it both sets MRU mode for tabs, and also enables tab previews when pressed (or pressed > once). Disabling it disables MRU mode, and also disables tab previews.
But what isn't possible, is to combine MRU tab switching, with absence of tab previews, because the way this preference works, means you get both or neither.
Can this preference be split into 2 separate preferences: browser.ctrlTab.previews and browser.ctrlTab.MRU_switching?
It's probably a very simple enhancement - and the updater could ensure no user surprises by testing whether browser.ctrlTab.MRU_switching exists, and if not, create it and set it equal to browser.ctrlTab.previews (if it already exists, don't do anything).
Suggestions and feature requests for the Pale Moon browser