How to install the English Thesaurus?

English
,
#1

The thesaurus option Ctrl F7 is grayed out. How can I activate it?

English thesaurus offers English USA words in preference
#2

A Dictionary extension that includes the Thesaurus for English (UK) can be downloaded here: https://www.weeklywhinge.com/files/en_GB_2_0_0.oxt

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#3

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