Having just seen the news that an author called Polly Courtney has fired her publishers for being sexist and generally patronising women with the covers etc, I'm not jumping completely to the defence of an industry that has found the dumb, frothy covers generally sell better. Certainly my mum's cousin, the author Miranda Glover, has a similar problem I believe and was badly hit on sales because of a trashy cover on a clever books that satisfied no-one.
HOWEVER, the contracts manager in me is screaming to point out that you don't really fire a publisher, you come to your end of your contract and don't sign a new one or you refuse to keep to the terms of one and run the risk of getting sued.
BUT what you might do, if you reckon your next book will be better and the imprint who publish you is a lower-tier list that offers poorer royalty terms than a standard list (such as some consider Avon to be) you might want to drum up some publicity in said next book and publicly 'fire' them. Whether or not they're sexist. Just sayin.