Try to change your home page on the settings ... I do not think it is an add-on

You didn't post your installed add-on in More System Details, so I can't make an "educated guess".

What add-ons do you have installed where the Last Updated (which can be seen when you click 'More' for each extension) corresponds with when that began.

Also, you may need to uninstall a program in the Windows add or Remove Programs list. That might be a severe infection.
http://www.bleepingcomputer.com/forums/t/496917/infected-with-startiminent-virus-browser-hijack/

It may also be worth looking at the shortcut used to start Firefox has that been added there so that Firefox opens the website when it is run.

If you type into the locationbar about:home and then hit the keyboard return key or click the green arrow

• do you see that page ?
• or do you see a standard Firefox homepage with a large Firefox icon above the centre page search bar ?

Many thanks!

Many thanks!