What do I do I really need this add on
Open Firefox and go to Tools Add-ons.
You'll find it in there, if there is no solution from Mendeley Support (which you will have to ask) then uninstall/remove that add-on.
This has nothing to do with McAfee really.
Solution otherwise would be to use another browser, or find out how to return Firefox to an earlier version and then turn off the autro-updating feature.
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