Re: I downloaded the add-on that allows you to have your homepage when you open a new tab. However it still hasn't changed to google, and I have restarted Firefox?
hello, this looks like a problem possibly caused by adware on your pc. please go to firefox > addons > extensions & remove any suspicious entries (toolbars, things that you have not installed intentionally, don't know what purpose they serve, etc). also go to the windows control panel / programs and remove all toolbars or potentially unwanted software from there.
also, you don't need an addon to change the page that shows up on a new tab. to do this manually, enter about:config into the firefox location bar (confirm the info message in case it shows up), search for the preference named browser.newtab.url and double-click & change its value to www.google.com (or whatever site you want).
& finally -this also needs your attention: please update your plugins (some of them are out-dated & have security vulnerabilities that are actively exploited on the web).