Depending on what was changed, check the following:

To set the home page to one of your choosing, see how to set the home page

There are several preferences that may have been changed.

1. Type about:config into the location bar and press enter
2. Accept the warning message that appears, you will be taken to a list of preferences
3. In the filter box type bing
4. For any preference with a value that include bing, right-click on it and choose Reset
5. If the above did not include the preference keyword.URL, locate that preference and right-click on it and choose Reset to set it back to Google

I do not have a tool bar or a location bar when I am in Firefox, only Bing. I typed "about:config" in the Bing box and did not get a list of preferences, only websites. I am sorry I don't know where to look for preferences.

Firefox used to work so perfectly for me, what's going on?

You should type about:config into the the same place that you type URLs for when you want to access a site.