Since I just wasted the better part of an hour on this problem, thought I'd share the solution I found.
In the process of setting up comcast service, I had to install software that added a very unhelpful Xfinity Search toolbar to firefox. At least that's what I thought happened. When I typed a search in the location bar (as I'm wont to do for a lazy shortcut), the search redirected to an Xfinity search page. How annoying!
Worse yet, even after I removed the Xfinity search engine from the browser (very easy to do with "Manage Search Engines"), searches continued to be redirected to the Xfinity page. I tried looking for Xfinity or Comcast add-ons in the browser and in the Windows Control Panel - nothing! I tried uninstalling and reinstalling firefox, but that didn't fix it. Aaaaargh! Then I tried Chrome, and the same thing happened there - Screeech!
Eventually I figured out that comcast - bless their hearts - does not return a "no-such-domain" error when you type a search in the location box, which is typically what tells your browser to redirect to your default search engine. Instead, comcast automatically redirects your search, via so-called DNS Assistance, to their own search page. (It was probably meant well, but it drove me bonkers.)
So - if you find yourself unexpectedly landing on the Xfinity search page when don't want to, here's what to do:
-- In Firefox, you have to update one of the advanced defaults using the following directions:
-- In Chrome, go to your Settings (chrome://chrome/settings/); under Search, you can specify the default search engine for input in the 'omnibox', plus you can remove any search providers you don't want with Manage Search Engines
Forgot to specify: to make the Firefox location bar work redirect to a search provider of your choice, you need to go to about:config in Firefox (type that in the location bar), then update the keyword:URL value to the search terms corresponding to your search provider. For example, to use Google web search as your default, set the keyword:URL setting to: