The default action when opening a new tab is to show a blank page. Something you installed changed that to Yahoo.

New Tab Homepage extension:
https://addons.mozilla.org/en-US/firefox/addon/777

Your UserAgent string in Firefox is messed up by another program that you installed, something from Comcast added NET_mmhpset. That change could stop websites from knowing you have Firefox 3.6.10 installed - which isn't the latest version - 3.6.12, you should update Firefox.
http://en.wikipedia.org/wiki/User_Agent

type about:config in the URL bar and hit Enter
If you see the warning, you can confirm that you want to access that page.
Filter = general.useragent.extra.firefox
Right-click that preference and select Reset
Then restart Firefox

Also, you have a number of plugins that are out of date. See this page.
http://www.mozilla.com/en-US/plugincheck/

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).

• Don't make any changes on the Safe mode start window.

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

Other things that need attention.
See http://www.mozilla.com/plugincheck/

Your above posted system details show outdated plugin(s) with known security and stability risks that you should update.

• Next Generation Java Plug-in 1.6.0_17 for Mozilla browsers

Update the Java plugin to the latest version.

• http://www.oracle.com/technetwork/java/javase/downloads/index.html (Java Platform: Download JRE)