Firefox 17.0.7esr?

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http://www.linuxquestions.org – I just ran slackpkg... which I do weekly... and found that the Firefox package being offered now is version 17.0.7esr. I'm confused because I was just upgraded to version 21.0 during my last update. I've tried multiple times to subscribe to the mailing lists but I've been unsuccessful so I'm kinda out of the loop here. If somebody could please just quickly explain this to me I would be very grateful. Thank you very much. (HowTos)