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Has worked rather well so far, however over the last few days YouTube seem to have changed something, as is their wont, so now the alert appears briefly every time again. Would you please look into it. Edited: the alert appears briefly on all sites.

Это отзыв для предыдущей версии этого дополнения (1.0-signed). 

Use version 2.0
https://addons.mozilla.org/ru/firefox/addon/hide-fulscreen-alert/versions/